When using -pg targeting OS X 10.8, pass -no_new_main to the linker.

By default on OS X 10.8, we don't link with a crt1.o file and the linker
knows to use _main as the entry point.  But, when compiling with -pg, we
need to link with the gcrt1.o file, and the linker needs to be told to use
the "start" symbol as the entry point.  The -no_new_main linker option does
that last part.  <rdar://problem/11491405>

llvm-svn: 159683