libdispatch#162: Android support

Hi guys,

I've been working on the following PR to enable cross-compilation for
Android armv7, in a similar way as it's being done in swift project

My idea was to bring a change that can be streamlined in swift's
compilation process easily, taking the same set of options. This should be
modified in the future when swift add support for multiple android archs.

I'm not sure how far from acceptable is this changeset, but let me know how
it can be improved. There are some potential issues I identified, where my
inexperience on libtools and cross compilation processes couldn't take me
to figure out a better solution.

Any comment is appreciated. Thanks!