Get rid of the android-cross-path flag to rustc.

It's a bit odd to single out just android when we don't do this for any other cross compiling targets. Android builds will still work since we just pass the full path for gcc and ar with `-C linker` and `-C ar`.

I did add the flag to compiletest though so it can find gdb. Though, i'm pretty sure we don't run debuginfo tests on android anyways right now.