RISC-V: Change constexpr back to CONSTEXPR

RISC-V: Change constexpr back to CONSTEXPR

According to https://github.com/gcc-mirror/gcc/commit/f95d3d5de72a1c43e8d529bad3ef59afc3214705.
Since GCC 4.8.6 doesn't support constexpr, we should change it back to CONSTEXPR.
gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: Change constexpr back to CONSTEXPR.
	* config/riscv/riscv-vector-builtins-shapes.cc (SHAPE): Ditto.
	* config/riscv/riscv-vector-builtins.cc (struct registered_function_hasher): Ditto.
	* config/riscv/riscv-vector-builtins.h (struct rvv_arg_type_info): Ditto.