Remove BFD_FAIL in cpu-sh.c

The assertions in cpu-sh.c can be triggered by passing bogus values
in disassemble_info.mach.  This doesn't cause any bfd misbehaviour.

	* cpu-sh.c (sh_get_arch_from_bfd_mach): Remove BFD_FAIL.
	(sh_get_arch_up_from_bfd_mach): Likewise.