[LPM] Try again to appease powerpc64 in its self host. I've been unable

[LPM] Try again to appease powerpc64 in its self host. I've been unable
to get a powerpc64 host so that I can reproduce and test this, but it
only impacts that platform so trying the only other realistic option.

According to Ulrich, who debugged this initially, initial-exec is likely
to be sufficient for our needs and not subject to this bug. Will watch
the build bots to see.

If this doesn't work, I'll be forced to cut a really ugly pthread-based
approach into the primary user (our stack trace printing) as that user
cannot use the ThreadLocal implementation due to lifetime issues.

llvm-svn: 227414