Specify -mtriple=x86_64 in an X86-specific dwarf test

On the PPC bot, the %llc_dwarf substitution does not contain an -mtriple
argument. This can cause the wrong backend to be exercised.

This causes issues because the backends differ in when they decide to
emit tail calls:

  http://lab.llvm.org:8011/builders/clang-ppc64be-linux-multistage/builds/12440

This is mostly a speculative fix as I don't have a PPC machine to test
with.

llvm-svn: 343893