Fix the androideabi toolchain not matching host

The included method of determining the host platform in the androideabi
would not match to windows if RUBY_PLATFORM returned mingw, wince, or
emx. As an added pre-caution, using a case statement with regexp
matching will better determine the host platform since it won't be case
sensitive and for windows is set to match more specifially than just
"win".
Merge pull request #1356 from CaptainJet/master

Fix matching the host platform on the androideabi toolchain on windows.