Fix ELF ppc32 targets that don't use ppc32elf.em

Fix ELF ppc32 targets that don't use ppc32elf.em

5446cbdf82892a800ed7eef563a795e75223ec52 broke powerpc-lynxos,
powerpc-netware, powerpc-windiss and powerpc-vxworks.

bfd/
	* elf32-ppc.c (ppc_elf_link_hash_table_create): Provide default
	params for targets that don't use ppc32elf.em.
ld/
	* emulparams/elf32ppcvxworks.sh: Source plt_unwind.sh and
	use ppc32elf.em.
	* emultempl/ppc32elf.em (ppc_after_open): Don't compile for
	vxworks.
	(LDEMUL_AFTER_OPEN): Don't set for vxworks.
	(PARSE_AND_LIST_LONGOPTS, PARSE_AND_LIST_OPTIONS): Exclude
	-secure-plt, -bss-plt and -sdata-got when vxworks.