Remove magic treatment of toc symbols for powerpc ELF

The XCOFF assembler does some wierd things with instructions like
`lwz 9,sym(30'.  See the comment in md_apply_fix.  From an ELF
perspective, it's weird even to magically select a TOC16 reloc
when a symbol is in the TOC/GOT.  ELF assemblers generally use
modifiers like @toc to select relocs, so remove this "feature"
for ELF.  I believe this was to support gcc -m32 -mcall-aixdesc
but that combination of gcc options has been broken for a long
time.

	* config/tc-ppc.c (ppc_is_toc_sym): Remove OBJ_ELF support.
	(md_assemble): Don't call ppc_is_toc_sym for ELF.
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.