Invoke the Darwin assembler with -g instead of --gdwarf2.

The -g and --gdwarf2 options are currently synonyms to the Darwin assembler.
But clang itself does not recognize --gdwarf2, so if we want to experiment
with using clang, with its integrated assembler, to replace the default
assembler, it is necessary to use -g.  <rdar://problem/10349486>

llvm-svn: 143533