RISC-V: Add ZVFH extension to the -march= option

This patch would like to add new sub extension (aka ZVFH) to the -march= option.
To make it simple, only the sub extension itself is involved in this patch, and
the underlying FP16 related RVV intrinsic API depends on the TARGET_ZVFH.

The Zvfh extension depends on the Zve32f and Zfhmin extensions. You can locate
more information about ZVFH from below spec doc.

https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#185-zvfh-vector-extension-for-half-precision-floating-point



Signed-off-by: Pan Li <pan2.li@intel.com>

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc:
	(riscv_implied_info): Add zvfh item.
	(riscv_ext_version_table): Ditto.
	(riscv_ext_flag_table): Ditto.
	* config/riscv/riscv-opts.h (MASK_ZVFH): New macro.
	(TARGET_ZVFH): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-21.c: New test.
	* gcc.target/riscv/predef-27.c: New test.