RISC-V: Add _mu C++ overloaded intrinsics for load && viota && vid

Base on these:
https://github.com/riscv-non-isa/rvv-intrinsic-doc/issues/232
https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/233



Add _mu C++ overloaded intrinsics for load && viota && vid.

Co-authored-by: KuanLin Chen <best124612@gmail.com>

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: Add _mu overloaded intrinsics.
	* config/riscv/riscv-vector-builtins-shapes.cc (struct fault_load_def): Ditto.