RISC-V: Use get_typenode_from_name to get fixed-width integer type nodes

RISC-V: Use get_typenode_from_name to get fixed-width integer type nodes

[u]int32_t are using different type between glibc and newlib, so getting
those node by int or long type isn't portable way, I also update all
other fixed-width integer types to prevent this happened again in future :P

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins.cc (register_builtin_types):
	Use get_typenode_from_name to get fixed-width integer type
	nodes.
	* config/riscv/riscv-vector-builtins.def: Update define with
	fixed-width integer type nodes.