[MinGW] Implicitly add .exe suffix if not provided

GCC implicitly adds an .exe suffix if it is given an output file name,
but the file name doesn't contain a suffix, and there are certain
users of GCC that rely on this behaviour (and run into issues when
trying to use Clang instead of GCC). And MSVC's cl.exe also does the
same (but not link.exe).

However, GCC only does this when actually running on windows, not when
operating as a cross compiler.

As GCC doesn't have this behaviour when cross compiling, we definitely
shouldn't introduce the behaviour in such cases (as it would break
at least as many cases as this fixes).

Differential Revision: https://reviews.llvm.org/D71400