RISC-V: Replace CONSTEXPR with constexpr

RISC-V: Replace CONSTEXPR with constexpr

Move away from the pre-C++11 compatibility macro CONSTEXPR.
This patch is inspired by aarch64:
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603974.html.

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: Replace CONSTEXPR
	with constexpr throughout.
	* config/riscv/riscv-vector-builtins-shapes.cc (SHAPE): Likewise.
	* config/riscv/riscv-vector-builtins.cc
	(struct registered_function_hasher): Likewise.
	* config/riscv/riscv-vector-builtins.h (struct rvv_arg_type_info):
	Likewise.