Support more relocs on 16-bit insn fields

This patch allows gas to assemble a testcase like
	li 3,ext_sym
which oddly was not accepted while the following is OK:
	li 3,ext_sym@l

	* config/tc-ppc.c (md_assemble): Move code adjusting reloc types
	later.  Merge absolute and relative branch reloc selection.
	Generate 16-bit relocs for most 16-bit insn fields given a
	non-constant expression.