compiler-rt/test: Add a couple of convenience features for Android.

Add a ANDROID_SERIAL_FOR_TESTING CMake variable. This lets you
run the tests with multiple devices attached without having to set
ANDROID_SERIAL.

Add a mechanism for pushing files to the device. Currently most
sanitizers require llvm-symbolizer and the sanitizer runtime to
be pushed to the device. This lets the sanitizer make this happen
automatically before running the tests by specifying the paths in
the lit.site.cfg file.

Differential Revision: https://reviews.llvm.org/D56712

llvm-svn: 351260