[GOLD] PowerPC move plt indx_ out of unordered map key

[GOLD] PowerPC move plt indx_ out of unordered map key

I was lazy when adding indx_ to Plt_stub_ent.  The field isn't part of
the key, so ought to be part of the mapped type.  Make it so.

	* powerpc.cc (Plt_stub_key): Rename from Plt_stub_ent.  Remove indx_.
	(Plt_stub_key_hash): Rename from Plt_stub_ent_hash.
	(struct Plt_stub_ent): New.
	(Plt_stub_entries): Map from Plt_stub_key to Plt_stub_ent.  Adjust
	use throughout file.