I have been trying out x86_64 msys2 (a fork of cygwin) on Windows insider
preview builds on ARM64, which added support for x86_64 emulation.  I was
getting the notorious FAST_CWD warnings, which were interfering with
MINGW CMake.  Last night late, I went looking at the cause, and found some
code that attempted to disable FAST_CWD warnings, but only on i686, and
trying to look at GetNativeSystemInfo, which was lying.  I quickly put
together this patch, and it seems to work.

Note I did this late at night, with no real regard to investigating or
matching code style.  This patch is currently more in an RFC state, if the
approach looks OK, and I'd be grateful for any pointers on getting it into
shape for evental acceptance.

Thanks,
Jeremy