Better overflow checking for powerpc32 relocations

Similar to the powerpc64 patch, this improves overflow checking in
elf32-ppc.c.  Many reloc "howto" entries needed fixes, some just
cosmetic.

The patch also fixes the R_PPC_VLE_SDA21 reloc application code, which
was horribly broken.  In fact, it may still be broken since Power ISA
2.07 says e_li behaves as
   RT <- EXTS(li20 1:4 || li20 5:8 || li20 0 || li20 9:19)
where li20 is a field taken from bits 17..20, 11..15, 21..31 of the
instruction.  Freescale VLEPEM says differently, and I assume
correctly, that
   RT <- EXTS(li20 0:3 || li20 4:8 || li20 9:19)
The VLE_SDA21 relocation description matches this too.

Now the VLE_SDA21 relocation specifies in the case where e_addi16 is
converted to e_li for symbols in .PPC.EMB.sdata0 or .PPC.EMB.sbss0
(no base register), that the field is restricted to 16 bits, with the
sign bit being propagated to the top 4 bits.  I don't see the sense in
restricting the value like this, so have allowed the full 20 bit
signed value.  This of course is compatible with the reloc description
in that values in the 16 bit signed range will result in exactly the
same insn field as when the reloc description is followed to the
letter.

	* elf32-ppc.c (ppc_elf_howto_raw): Correct overflow check for
	many relocations.  Correct bitsize and rightshift too for a number
	of VLE relocs.  Describe R_PPC_VLE_SDA21 and R_PPC_VLE_SDA21_LO.
	Correct dst_mask on R_PPC_VLE_SDA21_LO.
	(ppc_elf_vle_split16): Tidy, delete unnecessary prototype.
	(ppc_elf_relocate_section): Modify overflow test for 16-bit
	fields in instructions to signed/unsigned according to whether
	the field takes a signed or unsigned value.  Tidy vle split16 code.
	Correct R_PPC_VLE_SDA21 and R_PPC_VLE_SDA21_LO handling.

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