rustbuild: Implement testing for Android

rustbuild: Implement testing for Android

This commit enhances the rustbuild support for testing Android to the same level
of parity as the makefiles. This involved:

* A new step to copy the standard library and other shared objects to the
  emulator. This is injected as a dependency of all test suites for Android.
* Appropriate arguments are now passed through to compiletest to ensure that it
  can run tests.
* When testing the standard library the test executables are probed for and
  shipped to the emulator to run for each test.
* Fixing compilation of compiler-rt a bit

All support added here is modeled after what's found in the makefiles, just
translating one strategy to another. As an added bonus this commit adds support
for the "check" step to automatically run tests for all targets, and the
"check-target" step now runs all tests for a particular target, automatically
filtering the tests if the target is detected as a cross-compile.

Note that we don't (and probably won't) have a bot which is actually going to
exercise any of this just yet, but all tests have passed locally for me at
least.