Fix gdbserver build for x86-64 Windows

Fix gdbserver build for x86-64 Windows

I broke the gdbserver build on x86-64 Windows a little while back.
Previously, I could not build this configuration, but today I found
out that if I configure with:

    --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32

using the Fedora 34 tools, it will in fact build.  I'm not certain,
but maybe the gnulib update helped with this.

This patch fixes the build.  I'm checking it in.