RISC-V: Support Zmmul extension

RISC-V: Support Zmmul extension

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: Add Zmmul.
	* config/riscv/riscv-opts.h (MASK_ZMMUL): New.
	(TARGET_ZMMUL): Ditto.
	* config/riscv/riscv.cc (riscv_option_override):Ditto.
	* config/riscv/riscv.md: Add Zmmul
	* config/riscv/riscv.opt: Ditto.

gcc/testsuite/ChangeLog:

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