libstdc++: Implement [simd] for C++26

This implementation differs significantly from the
std::experimental::simd implementation. One goal was a reduction in
template instantiations wrt. what std::experimental::simd did.

Design notes:

- bits/vec_ops.h contains concepts, traits, and functions for working
  with GNU vector builtins that are mostly independent from std::simd.
  These could move from std::simd:: to std::__vec (or similar). However,
  we would then need to revisit naming. For now we kept everything in
  the std::simd namespace with __vec_ prefix in the names. The __vec_*
  functions can be called unqualified because they can never be called
  on user-defined types (no ADL). If we ever get simd<UDT> support this
  will be implemented via bit_cast to/from integral vector
  builtins/intrinsics.

- bits/simd_x86.h extends vec_ops.h with calls to __builtin_ia32_* that
  can only be used after uttering the right GCC target pragma.

- basic_vec and basic_mask are built on top of register-size GNU vector
  builtins (for now / x86). Any larger vec/mask is a tree of power-of-2
  #elements on the "first" branch. Anything non-power-of-2 that is
  smaller than register size uses padding elements that participate in
  element-wise operations. The library ensures that padding elements
  lead to no side effects. The implementation makes no assumption on the
  values of these padding elements since the user can bit_cast to
  basic_vec/basic_mask.

Implementation status:

- The implementation is prepared for more than x86 but is x86-only for
  now.

- Parts of [simd] *not* implemented in this patch:

  - std::complex<floating-point> as vectorizable types
  - [simd.permute.dynamic]
  - [simd.permute.mask]
  - [simd.permute.memory]
  - [simd.bit]
  - [simd.math]
  - mixed operations with vec-mask and bit-mask types
  - some conversion optimizations (open questions wrt. missed
    optimizations in the compiler)

- This patch implements P3844R3 "Restore simd::vec broadcast from int",
  which is not part of the C++26 WD draft yet. If the paper does not get
  accepted the feature will be reverted.

- This patch implements D4042R0 "incorrect cast between simd::vec and
simd::mask via conversion to and from impl-defined vector types" (to be
published once the reported LWG issue gets a number).

- The standard feature test macro __cpp_lib_simd is not defined yet.

Tests:

- Full coverage requires testing
  1. constexpr,
  2. constant-propagating inputs, and
  3. unknown (to the optimizer) inputs
  - for all vectorizable types
  * for every supported width (1–64 and higher)
  + for all possible ISA extensions (combinations)
  = with different fast-math flags
  ... leading to a test matrix that's far out of reach for regular
  testsuite builds.

- The tests in testsuite/std/simd/ try to cover all of the API. The
  tests can be build in every combination listed above. Per default only
  a small subset is built and tested.

- Use GCC_TEST_RUN_EXPENSIVE=something to compile the more expensive
  tests (constexpr and const-prop testing) and to enable more /
  different widths for the test type.

- Tests can still emit bogus -Wpsabi warnings (see PR98734) which are
  filtered out via dg-prune-output.

Benchmarks:

- The current implementation has been benchmarked in some aspects on
  x86_64 hardware. There is more optimization potential. However, it is
  not always clear whether optimizations should be part of the library
  if they can be implemented in the compiler.

- No benchmark code is included in this patch.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add simd headers.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def (simd): New.
	* include/bits/version.h: Regenerate.
	* include/bits/simd_alg.h: New file.
	* include/bits/simd_details.h: New file.
	* include/bits/simd_flags.h: New file.
	* include/bits/simd_iterator.h: New file.
	* include/bits/simd_loadstore.h: New file.
	* include/bits/simd_mask.h: New file.
	* include/bits/simd_mask_reductions.h: New file.
	* include/bits/simd_reductions.h: New file.
	* include/bits/simd_vec.h: New file.
	* include/bits/simd_x86.h: New file.
	* include/bits/vec_ops.h: New file.
	* include/std/simd: New file.
	* testsuite/std/simd/arithmetic.cc: New test.
	* testsuite/std/simd/arithmetic_expensive.cc: New test.
	* testsuite/std/simd/create_tests.h: New file.
	* testsuite/std/simd/creation.cc: New test.
	* testsuite/std/simd/creation_expensive.cc: New test.
	* testsuite/std/simd/loads.cc: New test.
	* testsuite/std/simd/loads_expensive.cc: New test.
	* testsuite/std/simd/mask2.cc: New test.
	* testsuite/std/simd/mask2_expensive.cc: New test.
	* testsuite/std/simd/mask.cc: New test.
	* testsuite/std/simd/mask_expensive.cc: New test.
	* testsuite/std/simd/reductions.cc: New test.
	* testsuite/std/simd/reductions_expensive.cc: New test.
	* testsuite/std/simd/shift_left.cc: New test.
	* testsuite/std/simd/shift_left_expensive.cc: New test.
	* testsuite/std/simd/shift_right.cc: New test.
	* testsuite/std/simd/shift_right_expensive.cc: New test.
	* testsuite/std/simd/simd_alg.cc: New test.
	* testsuite/std/simd/simd_alg_expensive.cc: New test.
	* testsuite/std/simd/sse_intrin.cc: New test.
	* testsuite/std/simd/stores.cc: New test.
	* testsuite/std/simd/stores_expensive.cc: New test.
	* testsuite/std/simd/test_setup.h: New file.
	* testsuite/std/simd/traits_common.cc: New test.
	* testsuite/std/simd/traits_impl.cc: New test.
	* testsuite/std/simd/traits_math.cc: New test.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>