[compiler-rt] Fix COMPILER_RT_OS_DIR for Android

Android has its own CMAKE_SYSTEM_NAME, but the OS is Linux (Android
target triples look like aarch64-none-linux-android21). The driver will
therefore search for compiler-rt libraries in the "linux" directory and
not the "android" directory, so the default placement of Android
compiler-rt libraries was incorrect. You could fix it by specifying
COMPILER_RT_OS_DIR manually, but it also makes sense to fix the default,
to save others from having to discover and fix the issue for themselves.