[GOLD] Avoid duplicate PLT stub symbols on ppc32

[GOLD] Avoid duplicate PLT stub symbols on ppc32

If two objects are compiled with -fPIC or -fPIE and call the same
function, two different PLT entries are created, one for each object,
but the same stub symbol name is used for both.

	* powerpc.cc (Stub_table::define_stub_syms): Always include object's
	uniq_ value.