aarch64: Add march flags for +fp8 arch extensions

aarch64: Add march flags for +fp8 arch extensions

This introduces the relevant flags to enable access to the fpmr register and fp8 intrinsics, which will be added subsequently.

gcc/ChangeLog:

	* config/aarch64/aarch64-option-extensions.def (fp8): New.
	* config/aarch64/aarch64.h (TARGET_FP8): Likewise.
	* doc/invoke.texi (AArch64 Options): Document new -march flags
	and extensions.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/acle/fp8.c: New test.