RISC-V: Add riscv-*-* configure support, and minor cleanup.

RISC-V: Add riscv-*-* configure support, and minor cleanup.

	bfd/
	* config.bfd (riscv32*-*-*): Renamed from riscv32-*-*.
	(riscv64*-*-*): Likewise.
	(riscv-*-*): Add as an alias for riscv32*-*-*.

	ld/
	* configure.tgt (riscv-*-*): Add as an alias for riscv32*-*-*.