This patch add the TLS Global-Dynamic to Initial-Exec TLS optimization
for AArch64. Similar of GD to LE, the idea is to optimize TLS access
to global symbols that can preempted by emitting a GOT entry and
a dynamic relocation (R_AARCH64_TLS_TPREL64).
This patch is based upon http://reviews.llvm.org/D18331
lowercase please:
llvm_unreachable("unsupported relocation for TLS GD to IE relaxation");