[ELF/AArch64] Initial General-dynamic TLS support

This patch adds initial general-dynamic TLS support for AArch64.  Currently
no optimization is done to realx for more performance-wise models (initial-exec
or local-exec).  This patch also only currently handles correctly executable
generation, although priliminary DSO support through PLT specific creation
is also added.

With this change clang/llvm bootstrap with lld is possible in static configuration
(some DSO creation fails due missing Linker script support, not AArch64 specific),
although make check also shows some issues.

llvm-svn: 241192