aarch64: PR target/115475 Implement missing __ARM_FEATURE_SVE_BF16 macro

aarch64: PR target/115475 Implement missing __ARM_FEATURE_SVE_BF16 macro

The ACLE requires __ARM_FEATURE_SVE_BF16 to be enabled when SVE and BF16
and the associated intrinsics are available.
GCC does support the required intrinsics for TARGET_SVE_BF16 so define
this macro too.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/

	PR target/115475
	* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins):
	Define __ARM_FEATURE_SVE_BF16 for TARGET_SVE_BF16.

gcc/testsuite/

	PR target/115475
	* gcc.target/aarch64/acle/bf16_sve_feature.c: New test.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>
(cherry picked from commit 6492c7130d6ae9992298fc3d072e2589d1131376)