expand: Increase stack slot alignment for STRICT_ALIGNMENT targets [PR123447]

PR123447 reports an ICE on AArch64 with "-O2 -mstrict-align" in subreg
lowering while decomposing the following multiword store RTL:

(insn 12 11 13 2 (set (mem/c:XI (plus:DI (reg/f:DI 64 sfp)
                (const_int -96 [0xffffffffffffffa0])) [0  S64 A8])
        (reg:XI 103)) "a.c":14:6 4861 {*aarch64_movxi}

This RTL originates from expanding the following GIMPLE statement:

_1 = BIT_FIELD_REF <{ 9, -64497, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }, 256, 0>;

The operand is a constant _Decimal64 vector with BLKmode, so expand has
to materialize it in memory. Current get_object_alignment() returns a
1-byte guaranteed alignment for this VECTOR_CST, as indicated by A8 in
the RTL dump above. However, with "-mstrict-align" enabled, later subreg
lowering pass expects at least 64-bit alignment when it constructs a new
RTX to decompose the store into pieces. Because the original alignment
is too small, simplify_gen_subreg() returns NULL_RTX and an assertion is
hit.

This patch increases the stack slot alignment for STRICT_ALIGNMENT
targets, when the operand is forced into memory. The increased alignment
is capped by MAX_SUPPORTED_STACK_ALIGNMENT so it won't be too large.

Bootstrapped and tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/ChangeLog:

	PR middle-end/123447
	* expr.cc (expand_expr_real_1): Increase stack slot alignment
	for STRICT_ALIGNMENT targets.

gcc/testsuite/ChangeLog:

	PR middle-end/123447
	* gcc.dg/pr123447.c: New test.