RISC-V: Minimal support for ssnpm, smnpm and smmpm extensions.

RISC-V: Minimal support for ssnpm, smnpm and smmpm extensions.

This patch support ssnpm, smnpm, smmpm, sspm and supm extensions[1].
To enable GCC to recognize and process ssnpm, smnpm, smmpm, sspm and
supm extensions correctly at compile time.

[1]https://github.com/riscv/riscv-j-extension/blob/master/zjpm/instructions.adoc

Changes for v5:
- Fix the testsuite error in arch-50.c.
Changes for v4:
- Fix the code based on the commit id 9b13bea07706a7cae0185f8a860d67209308c050.
Changes for v3:
- Fix the error messages in gcc/testsuite/gcc.target/riscv/arch-46.c
Changes for v2:
- Add the sspm and supm extensions.
- Add the check_conflict_ext function to check the compatibility of ssnpm, smnpm, smmpm, sspm and supm extensions.
- Add the test cases for ssnpm, smnpm, smmpm, sspm and supm extensions.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc
	(riscv_subset_list::check_conflict_ext): New extension.
	* config/riscv/riscv.opt: Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-ss-1.c: New test.
	* gcc.target/riscv/arch-ss-2.c: New test.