[ELF][PPC64] Don't apply LD->LE relaxation on R_PPC64_GOT_DTPREL16*

In ELF v2 ABI, R_PPC64_GOT_DTPREL16* are not relaxed.

This family of relocation types are used for variables outside of 2GiB
of the TLS block. 2 instructions cannot materialize a DTPREL offset that
is not 32-bit.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62737

llvm-svn: 362357