aarch64: PR target/115457 Implement missing __ARM_FEATURE_BF16 macro

aarch64: PR target/115457 Implement missing __ARM_FEATURE_BF16 macro

The ACLE asks the user to test for __ARM_FEATURE_BF16 before using the
<arm_bf16.h> header but GCC doesn't set this up.
LLVM does, so this is an inconsistency between the compilers.

This patch enables that macro for TARGET_BF16_FP.
Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/

	PR target/115457
	* config/aarch64/aarch64-c.cc (aarch64_update_cpp_builtins):
	Define __ARM_FEATURE_BF16 for TARGET_BF16_FP.

gcc/testsuite/

	PR target/115457
	* gcc.target/aarch64/acle/bf16_feature.c: New test.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>