aarch64: Add support for _BitInt

aarch64: Add support for _BitInt

This patch adds support for C23's _BitInt for the AArch64 port when compiling
for little endianness.  Big Endianness requires further target-agnostic
support and we therefor disable it for now.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (TARGET_C_BITINT_TYPE_INFO): Declare MACRO.
	(aarch64_bitint_type_info): New function.
	(aarch64_return_in_memory_1): Return large _BitInt's in memory.
	(aarch64_function_arg_alignment): Adapt to correctly return the ABI
	mandated alignment of _BitInt(N) where N > 128 as the alignment of
	TImode.
	(aarch64_composite_type_p): Return true for _BitInt(N), where N > 128.

libgcc/ChangeLog:

	* config/aarch64/t-softfp (softfp_extras): Add floatbitinthf,
	floatbitintbf, floatbitinttf and fixtfbitint.
	* config/aarch64/libgcc-softfp.ver (GCC_14.0.0): Add __floatbitinthf,
	__floatbitintbf, __floatbitinttf and __fixtfbitint.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/bitint-alignments.c: New test.
	* gcc.target/aarch64/bitint-args.c: New test.
	* gcc.target/aarch64/bitint-sizes.c: New test.
	* gcc.target/aarch64/bitfield-bitint-abi.h: New header.
	* gcc.target/aarch64/bitfield-bitint-abi-align16.c: New test.
	* gcc.target/aarch64/bitfield-bitint-abi-align8.c: New test.