Raw.xs: handle absolute paths correctly on Cygwin

In cygwin land there are POSIX and windows paths, LoadLibrary requires a windows
path since it is not a cygwin function, but FFI::Raw -> new() should be able to
handle a absolute POSIX path, so for Cygwin only, convert the POSIX path to
windows before attempting LoadLibrary.
Changes: update
Raw.xs: use little-endian code for MSWin32 (x86)

Neither __BYTE_ORDER nor __BIG_ENDIAN seem to be defined on Windows, so it was
selecting the big-endian path instead of the intended little-endian path.
MakeMaker.pm: use the 'system lib' for MSWin32 too

When I originally did the system lib check, I didn't think I would be using
pkg-config on Windows, but I actaully managed to get it to work, and this will
save me a lot of time not having to run configure every time I need to run dzil
test.