Update Apple's build script for llvmCore. <rdar://problem/12914321>

This change is basically just copying changes that we've used for Apple's
clang builds to the script used for building llvmCore. Besides cleaning it
up to use xcrun to locate the proper versions of tools, especially for cross
compiling, it fixes the build to work with newer versions of clang that
honor SDKROOT settings in the environment.