[sanitizer] Android build cleanup.

* Detect Android toolchain target arch and set correct runtime library name.
* Merged a lot of Android and non-Android code paths.
* Android is only supported in standalone build of compiler-rt now.
* Linking lsan-common in ASan-Android (makes lsan annotations work).
* Relying on -fsanitize=address linker flag when building tests (again,
  unification with non-Android path).
* Runtime library moved from lib/asan to lib/linux.