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).
With this patch, the R_AARCH64_TLSDESC* implementation 
(still in review), and a small one for aarch64 to fix the relativeRel
definition (which I plan to send for review shortly) I can bootstrap
and run all the lld tests without failure.