Workaround a C stack overflow on MinGW64.

The hlc.gc version of the Mercury compiler exhausts the C stack while compiling
the standard library.  This is because the default size of the C stack on
Windows is ridiculously low.

compiler/Mmakefile:
    On the x86_64-w64-mingw32 architecture, bump the default size
    of the C stack up to something more reasonable.

    XXX in the long term, i.e. when we support 64-bit MSVC, we
    should add a --stack option to the ml script and use that.
    (The above is specific to GCC.)