PowerPC PLT stub alignment fixes

PowerPC PLT stub alignment fixes

Asking for ppc32 plt call stubs to be aligned at 32 byte boundaries
didn't quite work.  For ld.bfd they were spaced 32 bytes apart, but
only started on a 16 byte boundary.  ld.gold also didn't get it right.

Finding that bug made me check over the ppc64 plt stub alignment,
where I found that negative values for alignment (meaning align to
minimize boundary crossing) were not accepted.  Since no one has
complained about that, I guess I could have removed the feature from
ld.bfd documentation, but I've opted instead to correct the code.

I've also added an optional alignment paramenter for ppc32
--plt-align, for some consistency with gold and ppc64 ld.bfd.

bfd/
	* elf32-ppc.c (ppc_elf_create_glink): Correct alignment of .glink.
	* elf64-ppc.c (ppc64_elf_size_stubs): Handle negative plt_stub_align.
	(ppc64_elf_build_stubs): Likewise.
gold/
	* powerpc.cc (param_plt_align): New function supplying default
	--plt-align values.  Use it..
	(Stub_table::plt_call_align): ..here, and..
	(Output_data_glink::global_entry_align): ..here.
	(Stub_table::stub_align): Correct 32-bit minimum alignment.
ld/
	* emultempl/ppc32elf.em: Support optional --plt-align arg.
	* emultempl/ppc64elf.em: Support negative --plt-align arg.