rtld-elf: Fix powerpc64 TLS handling, matching powerpc's fix

We need to subtract the TLS_TCB_SIZE to get to the real data pointer, since
r13 points to the end of the TCB structure. Prior to this, devel/protobuf-c
port broke with recent update to devel/protobuf, which exposed this issue.

Submitted by: andreast
Reported by: Piotr Kubaj
MFC after: 1 week