RISC-V: Fix gdbserver problem with handling arch strings.

Maciej reported a problem found by his RISC-V gdbserver port.
warning: while parsing target description (at line 4): Target description specified unknown architecture "riscv:rv64id"
warning: Could not load XML target description; ignoring

We only have two arches defined, riscv:rv32 and riscv:rv64.  Both bfd and
gdb are creating arch strings that have extension letters added to the base
architecture.  The bfd_default_scan function requires an exact match, so
these strings fail to map to a bfd_arch.  I think we should ignore the
extension letters in a RISC-V specific scan function.

	bfd/
	* cpu-riscv.c (riscv_scan): New.
	(N): Change bfd_default_scan to riscv_scan.

Change-Id: I096476705e1da5cb8934c5005b1eed2a8989f7a7