Use CreateSemaphoreW instead of CreateSemaphoreA in libgcc.

[Jacek Caban]

This is no-op for usual GCC targets, because we don't pass any string to
CreateSemaphore anyway. However this trivial change will help
mingw-w64's efforts to support WinRT, where only unicode variant is
available.

libgcc/Changelog:
    config/i386/gthr-win32.c: CreateSemaphoreW instead of CreateSemaphoreA.
    config/i386/gthr-win32.h: Likewise.