[GOLD, PowerPC] Add --no-stub-group-multi

[GOLD, PowerPC] Add --no-stub-group-multi

The trouble with stubs per output section is that ppc32 uses a plt
stub as the address of a global function.  This needs to be unique,
otherwise we'll get multiple addresses for a function.

Obviously this is only a partial solution, since ppc32 will get
multiple stubs when code is larger than 33M.  A proper fix will
involve selecting a unique stub to use for non-branch relocs.

	* options.h (stub-group-multi): Default to true.  Add
	--no-stub-group-multi.