As is described at http://llvm.org/bugs/show_bug.cgi?id=22408, the GNU
linkers ld.bfd and ld.gold currently only support a subset of the
whole range of AArch64 ELF TLS relocations. Furthermore, they assume
that some of the code sequences to access thread-local variables are
produced in a very specific sequence.  When the sequence is not as the
linker expects, it can silently mis-relaxe/mis-optimize the
instructions.
Even if that wouldn't be the case, it's good to produce the exact
sequence, as that ensures that linkers can perform optimizing
relaxations.

This patch:

* implements support for 16MiB TLS area size instead of 4GiB TLS area
  size. Ideally clang would grow an -mtls-size option to allow support
  for both, but that's not part of this patch.
* by default doesn't produce local dynamic access patterns, as even
  modern ld.bfd and ld.gold linkers do not support the associated
  relocations. An option (-aarch64-elf-ldtls-generation) is added to
  enable generation of local dynamic code sequence, but is off by
  default.
* makes sure that the exact expected code sequence for local dynamic
  and general dynamic accesses is produced, by making use of a new
  pseudo instruction. The patch also removes two
  (AArch64ISD::TLSDESC_BLR, AArch64ISD::TLSDESC_CALL) pre-existing
  AArch64-specific pseudo SDNode instructions that are superseded by
  the new one (TLSDESC_CALLSEQ).