RISC-V: Implement TARGET_COMPARE_VERSION_PRIORITY and TARGET_OPTION_FUNCTION_VERSIONS

This patch implements TARGET_COMPARE_VERSION_PRIORITY and
TARGET_OPTION_FUNCTION_VERSIONS for RISC-V.

The TARGET_COMPARE_VERSION_PRIORITY is implemented to compare the
priority of two function versions based on the rules defined in the
RISC-V C-API Doc PR #85:

https://github.com/riscv-non-isa/riscv-c-api-doc/pull/85/files#diff-79a93ca266139524b8b642e582ac20999357542001f1f4666fbb62b6fb7a5824R721



If multiple versions have equal priority, we select the function with
the most number of feature bits generated by
riscv_minimal_hwprobe_feature_bits. When it comes to the same number of
feature bits, we diff two versions and select the one with the least
significant bit set. Since a feature appears earlier in the feature_bits
might be more important to performance.

The TARGET_OPTION_FUNCTION_VERSIONS is implemented to check whether the
two function versions are the same. This Implementation reuses the code
in TARGET_COMPARE_VERSION_PRIORITY and check it returns 0, which means
the equal priority.

Co-Developed-by: Hank Chang <hank.chang@sifive.com>
Signed-off-by: Yangyu Chen <cyy@cyyself.name>

gcc/ChangeLog:

	* config/riscv/riscv.cc
	(parse_features_for_version): New function.
	(compare_fmv_features): New function.
	(riscv_compare_version_priority): New function.
	(riscv_common_function_versions): New function.
	(TARGET_COMPARE_VERSION_PRIORITY): Implement it.
	(TARGET_OPTION_FUNCTION_VERSIONS): Implement it.