Make TLS work for PIE executables on aarch64.

Similar to r265462, TLS related relocations aren't marked as relative,
meaning that we end up generating R_AARCH64_RELATIVE relocations for
them. This change adds TLS relocations that I've seen on my system. With
this patch applied CloudABI's unit testing binary now passes on aarch64.

Approved by:	ruiu
Differential Revision:	http://reviews.llvm.org/D18816

llvm-svn: 265575