pdp11 reloc processing

This allows pdp11 to handle 32-bit fixups that can be applied by gas,
and improves the error message emitted when the required reloc isn't
available.

	* config/tc-pdp11.c (md_apply_fix): Handle BFD_RELOC_32.
	(tc_gen_reloc): Only give a BAD_CASE assertion on pcrel relocs.