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

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

This patch would like to add new sub extension (aka ZVFHMIN) 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_ZVFHMIN.

The Zvfhmin extension depends on the Zve32f extension. You can locate
more information about ZVFHMIN from below spec doc.

https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#zvfhmin-vector-extension-for-minimal-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 zvfhmin item.
	(riscv_ext_version_table): Ditto.
	(riscv_ext_flag_table): Ditto.
	* config/riscv/riscv-opts.h (MASK_ZVFHMIN): New macro.
	(TARGET_ZFHMIN): Align indent.
	(TARGET_ZFH): Ditto.
	(TARGET_ZVFHMIN): New macro.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-20.c: New test.
	* gcc.target/riscv/predef-26.c: New test.

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