[compiler-rt] Improve defaults for Android

Android has only used libc++ for a long time, and since NDK r23, it
also always uses compiler-rt and LLVM's libunwind (which is linked
statically). Reflect these defaults in compiler-rt's build, instead of
requiring the correct settings to always be externally specified.

Reviewed By: srhines

Differential Revision: https://reviews.llvm.org/D158792