RISC-V: Add rounding mode enum for fixed-point intrinsics

RISC-V: Add rounding mode enum for fixed-point intrinsics

Hi, since fixed-point with modeling rounding mode intrinsics are coming:
https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/222

I am adding vxrm rounding mode enum to user first before the API intrinsic.

This patch is simple && obvious.

Ok for trunk ?

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins.cc (register_vxrm): New function.
	(DEF_RVV_VXRM_ENUM): New macro.
	(handle_pragma_vector): Add vxrm enum register.
	* config/riscv/riscv-vector-builtins.def (DEF_RVV_VXRM_ENUM): New macro.
	(RNU): Ditto.
	(RNE): Ditto.
	(RDN): Ditto.
	(ROD): Ditto.

gcc/testsuite/ChangeLog:

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