TLS support for MinGW targets.
MinGW is almost completely compatible to MSVC, with the exception of the _tls_array global not being available.

Patch by David Nadlinger!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177257 91177308-0d34-0410-b5e6-96231b3b80d8