gccrs: intrinsic: Add size_of_val

This patch adds the size_of_val intrinsic to the compiler. This
intrinsic evaluates the size of a value at runtime, including
Dynamically Sized Types (DSTs).

gcc/rust/ChangeLog:

	* backend/rust-compile-intrinsic.cc (generic_intrinsics): Add
	size_of_val_handler to the map.
	* backend/rust-intrinsic-handlers.cc (size_of_val_handler): New
	function.
	* backend/rust-intrinsic-handlers.h (size_of_val_handler): New
	declaration.
	* typecheck/rust-hir-type-check-intrinsic.cc
	(IntrinsicChecker::intrinsic_rules): Add size_of_val signature
	rule to the map.
	* util/rust-intrinsic-values.h (class Intrinsics): Add
	SIZE_OF_VAL constexpr.

gcc/testsuite/ChangeLog:

	* rust/execute/size-of-val.rs: New test.

Signed-off-by: Enes Cevik <enes@nsvke.com>