llvm: Disable pthread on mingw

llvm supports both win32 native threads and pthread,
but configure tries to find pthread first.
This manually disables pthread to use native api.

This removes libpthreads-2.dll dependency on librustc.
mklldeps.py: Write to file instead of print

It seems that msys automatically converts `\n` to `\r\n` on pipe
redirection, which causes `make tidy` failure.
auto merge of #11013 : klutzy/rust/win-no-pthread, r=alexcrichton

llvm supports both win32 native threads and pthread,
but configure tries to find pthread first.
This manually disables pthread to use native api.

This removes libpthreads-2.dll dependency on librustc.