[LPM] Clean up the use of TLS in pretty stack trace and disable it

entirely when threads are not enabled. This should allow anyone who
needs to bootstrap or cope with a host loader without TLS support to
limp along without threading support.

There is still some bug in the PPC TLS stuff that is not worked around.
I'm getting access to a machine to reproduce and debug this further.
There is some chance that I'll have to add a terrible workaround for
PPC.

There is also some problem with iOS, but I have no ability to really
evaluate what the issue is there. I'm leaving it to folks maintaining
that platform to suggest a path forward -- personally I don't see any
useful path forward that supports threading in LLVM but does so without
support for *very basic* TLS. Note that we don't need more than some
pointers, and we don't need constructors, destructors, or any of the
other fanciness which remains widely unimplemented.

llvm-svn: 227411