RISC-V: Add RVV FRM enum for floating-point rounding mode intriniscs

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins.cc (register_frm): New function.
	(DEF_RVV_FRM_ENUM): New macro.
	(handle_pragma_vector): Add FRM enum
	* config/riscv/riscv-vector-builtins.def (DEF_RVV_FRM_ENUM): New macro.
	(RNE): Ditto.
	(RTZ): Ditto.
	(RDN): Ditto.
	(RUP): Ditto.
	(RMM): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/frm-1.c: New test.