[clang] Don't set CLANG_DEFAULT_UNWINDLIB to none if rtlib is set to compiler-rt

002dd47b was meant to not be any
functional change, but it turned out it was.

With CLANG_DEFAULT_RTLIB set to compiler-rt, CLANG_DEFAULT_UNWINDLIB used
to bet set to an empty string, but now was set to "none".

If one only overrode rtlib to libgcc, one previously would get libgcc
as unwind lib, but now didn't. This caused test failures, fixed in
41476d89.

Secondly, for the android target, the previous default was to link
libunwind, which this now changed.

Reinstate the exact same behaviour as before (removing the previously
typoed cmake check) and fix the option comment in one place to match
the other one above.

Differential Revision: https://reviews.llvm.org/D98142