[PPC64] Support relaxing R_PPC64_TLSGD16 in TlsGdtoLe relaxation.

A General-dynamic tls access can be written using a R_PPC64_TLSGD16 relocation
if the target got entry is within 16 bits of the TOC-base. This patch adds
support for R_PPC64_TLSGD16 by relaxing it the same as a R_PPC64_GOT_TLSGD16_LO.

Differential Revision: https://reviews.llvm.org/D52055

llvm-svn: 342411