Fix PR22408 - LLVM producing AArch64 TLS relocations that GNU linkers cannot handle...

Fix PR22408 - LLVM producing AArch64 TLS relocations that GNU linkers cannot handle yet.

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  SDNode instructions that are superseded by the new one (TLSDESC_CALLSEQ).

llvm-svn: 231227