[PowerPC] Add some special @got@tprel fixup cases

When a target@got@tprel or target@got@tprel@l symbol variant is used in
a fixup_ppc_half16 (*not* fixup_ppc_half16ds) context, we currently fail,
since the corresponding R_PPC64_GOT_TPREL16 / R_PPC64_GOT_TPREL16_LO
relocation types do not exist.

However, since such symbol variants resolve to GOT offsets which are
always 4-aligned, we can simply instead use the _DS variants of the
relocation types, which *do* exist.

The same applies for the @got@dtprel variants.