RISC-V: Add __RISCV_ prefix to VXRM and FRM enum

According to doc:
https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/222/files
https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/226

Add __RISCV_ prefix to VXRM and FRM enum.

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins.cc (DEF_RVV_VXRM_ENUM): Add
	__RISCV_ prefix.
	(DEF_RVV_FRM_ENUM): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/frm-1.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-1.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-10.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-11.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-12.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-6.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-7.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-8.c: Ditto.
	* gcc.target/riscv/rvv/base/vxrm-9.c: Ditto.