Revert "Support watchOS and tvOS in compiler-rt builds"

The required compiler-rt changes aren't present yet so attempting to
build with compiler-rt breaks. And since we're trying to deprecate
autotools we actually want to fix this in CMake primarily anyway.

This reverts r251712.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251953 91177308-0d34-0410-b5e6-96231b3b80d8