Fix arm bot failure after f91d18ea

llc doesn't seem to automatically pick default `--triple`.
using `%llc_dwarf` should fix this.

Builder:
http://lab.llvm.org:8011/builders/clang-cmake-armv7-quick/builds/20310

Error log:
bin/llc: error: : error: unable to get target for 'x86_64-unknown-linux-gnu', see --version and --triple.