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.
Adding tests for closed issues

Closes #5806
Closes #5950
Closes #7178
Closes #8259
Closes #8578
Closes #8851
Closes #9129
Closes #10412
auto merge of #11012 : alexcrichton/rust/needstest, r=alexcrichton

Closes #5806
Closes #8259
Closes #8578
Closes #8851
Closes #10412
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.