PowerPC @l, @h and @ha warnings, plus VLE e_li

PowerPC @l, @h and @ha warnings, plus VLE e_li

This patch started off just adding the warnings in tc-ppc.c about
incorrect usage of @l, @h and @ha in instructions that don't have
16-bit D-form fields.  That unfortunately showed up three warnings in
ld/testsuite/ld-powerpc/vle-multiseg.s on instructions like
	e_li r3, IV_table@l+0x00
which was being assembled to
   8:	70 60 00 00 	e_li    r3,0
			a: R_PPC_ADDR16_LO	IV_table
The ADDR16_LO reloc is of course completely bogus on e_li, which has
a split 20-bit signed integer field in bits 0x1f7fff, the low 11 bit
in 0x7ff, the next 5 bits in 0x1f0000, and the high 4 bits in 0x7800.
Applying an ADDR16_LO reloc to the instruction potentially changes
the e_li instruction to e_add2i., e_add2is, e_cmp16i, e_mull2i,
e_cmpl16i, e_cmph16i, e_cmphl16i, e_or2i, e_and2i., e_or2is, e_lis,
e_and2is, or some invalid encodings.

Now there is a relocation that suits e_li, R_PPC_VLE_ADDR20, which was
added 2017-09-05 but I can't see code in gas to generate the
relocation.  In any case, VLE_ADDR20 probably doesn't have the correct
semantics for @l since ideally you'd want an @l to pair with @h or @ha
to generate a 32-bit constant.  Thus @l should only produce a 16-bit
value, I think.  So we need some more relocations to handle e_li it
seems, or as I do in this patch, modify the behaviour of existing
relocations when applied to e_li instructions.

include/
	* opcode/ppc.h (E_OPCODE_MASK, E_LI_MASK, E_LI_INSN): Define.
bfd/
	* elf32-ppc.c (ppc_elf_howto_raw <R_PPC_VLE_ADDR20>): Correct
	mask and shift value.
	(ppc_elf_vle_split16): Use E_OPCODE_MASK.  Handle e_li
	specially.
gas/
	* config/tc-ppc.c (md_assemble): Adjust relocs for VLE before
	TLS tweaks.  Handle e_li.  Warn on unexpected operand field
	for lo16/hi16/ha16 relocs.