RISC-V: Support RISC-V Profiles 20/22.

RISC-V: Support RISC-V Profiles 20/22.

This patch introduces support for RISC-V Profiles RV20 and RV22 [1],
enabling developers to utilize these profiles through the -march option.

[1] https://github.com/riscv/riscv-profiles/releases/tag/v1.0

Version log:
Using lowercase letters to present Profiles.
Using '_' as divsor between Profiles and other RISC-V extension.
Add descriptions in invoke.texi.
Checking if there exist '_' between Profiles and additional extensions.
Using std::string to avoid memory problems.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (struct riscv_profiles): New struct.
	(riscv_subset_list::parse_profiles): New parser.
	(riscv_subset_list::parse_base_ext): Ditto.
	* config/riscv/riscv-subset.h: New def.
	* doc/invoke.texi: New option descriptions.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-49.c: New test.
	* gcc.target/riscv/arch-50.c: New test.
	* gcc.target/riscv/arch-51.c: New test.
	* gcc.target/riscv/arch-52.c: New test.