[compiler-rt] OS X: Update the CMake and Make builds to explicitely use...

[compiler-rt] OS X: Update the CMake and Make builds to explicitely use libc++, mmacosx-version-min and SDKs

In both CMake and Makefiles, we are inconsistent about the use of libstdc++ vs. libc++, SDKs and minimum deployment targets for OS X. Let's fix the detection of SDKs, and let's explicitely set that we link against libc++ and mmacosx-version-min is 10.7.

llvm-svn: 227509