std: prevent `CreateProcess()` race on Windows

std: prevent `CreateProcess()` race on Windows

Believe or not, `CreateProcess()` is racy if several threads create
child processes: [0], [1], [2].

This caused some tests show crash dialogs during
`make check-stage#-rpass`.

More explanation:

On Windows, `SetErrorMode()` controls display of error dialogs: it
accepts new error mode and returns old error mode.
The error mode is process-global and automatically inherited to child
process when created.

MSYS2 bash shell internally sets it to not show error dialogs, therefore
`make check-stage#-rpass` should not show them either.

However, [1] says that `CreateProcess()` internally invokes
`SetErrorMode()` twice: at first it sets mode `0x8001` and saves
original mode, and at second it restores original mode.
So if two threads simultaneously call `CreateProcess()`, the first
thread sets error mode to `0x8001` then the second thread recognizes
that current error mode is `0x8001`. Therefore, The second thread will
create process with wrong error mode.

This really occurs inside `compiletest`: it creates several processes on
each thread, so some `run-pass` tests are invoked with wrong error mode
therefore show crash dialog.

This commit adds `StaticMutex` for `CreateProcess()` call. This seems
to fix the "dialog annoyance" issue.

[0]: http://support.microsoft.com/kb/315939
[1]: https://code.google.com/p/nativeclient/issues/detail?id=2968
[2]: https://ghc.haskell.org/trac/ghc/ticket/2650