CMake: generalize checking for target availability and add initial support for PowerPC native arch. With this patch, building LLVM on PowerPC native arch produces a working ASan runtime.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173044 91177308-0d34-0410-b5e6-96231b3b80d8
[ASan] Move Mac-specific tests to asan_mac_test.cc



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173048 91177308-0d34-0410-b5e6-96231b3b80d8