Mingw 64 bit build

I've built nvm.msys2-mingw-64 with Mys2/MingW64 64 bit. It passes all tests
except one func test (empty_environment) and some extra tests.

I need to test that branch on a Unix box (I have Debian and Cygwin),
since most of the changes are in '#ifdef Windows' areas.

See INSTALL_windows_msys2_64.txt for tools install; there is also
INSTALL_windows_msys2_32.txt, which is very similar, but has "32"
instead of "64" in lots of places. Having two different files makes it
easier to cut and paste the commands.

-- 
-- Stephe