On OS X, explicitly use libc++

For OS X builds, both Make and CMake, let's be very explicit about using
libc++ and libc++abi with:

1) -stdlib=libc++ in CFLAGS and LDFLAGS for all platforms
2) -lc++ in LDFLAGS for all platforms
3) switch from -undefined dynamic_lookup to -lc++abi for UBSan in
   Makefile-based builds

Reviewed at http://reviews.llvm.org/D8617

llvm-svn: 233215