RISC-V: Minimal support for Zimop extension.

This patch support Zimop and Zcmop extension[1].To enable GCC to recognize
and process Zimop and Zcmop extension correctly at compile time.

https://github.com/riscv/riscv-isa-manual/blob/main/src/zimop.adoc

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: New extension.
	* config/riscv/riscv.opt: New mask.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-42.c: New test.
	* gcc.target/riscv/arch-43.c: New test.