[LPM] Try to work around a bug with local-dynamic TLS on PowerPC 64.

[LPM] Try to work around a bug with local-dynamic TLS on PowerPC 64.

Sadly, this precludes optimizing it down to initial-exec or local-exec
when statically linking, and in general makes the code slower on PPC 64,
but there's nothing else for it until we can arrange to produce the
correct bits for the linker.

Lots of thanks to Ulirch for tracking this down and Bill for working on
the long-term fix to LLVM so that we can relegate this to old host
clang versions.

I'll be watching the PPC build bots to make sure this effectively
revives them.

llvm-svn: 227352