RISC-V: Implement Priority syntax parser for Function Multi-Versioning

This patch adds the priority syntax parser to support the Function
Multi-Versioning (FMV) feature in RISC-V. This feature allows users to
specify the priority of the function version in the attribute syntax.

Chnages based on RISC-V C-API PR:
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/85



Signed-off-by: Yangyu Chen <cyy@cyyself.name>

gcc/ChangeLog:

	* config/riscv/riscv-target-attr.cc
	(riscv_target_attr_parser::handle_priority): New function.
	(riscv_target_attr_parser::update_settings): Update priority
	attribute.
	* config/riscv/riscv.opt: Add TargetVariable riscv_fmv_priority.