Fix arm bot failure after f91d18e

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.

Author: SouraVX

llvm/test/DebugInfo/distringtype.ll

@@ -5,7 +5,7 @@
 ;; !DIStringType(name: "character(*)", stringLength: !{{[0-9]+}},
 ;;		stringLengthExpression: !DIExpression(), size: 32)
 
-; RUN: llc -filetype=obj  %s -o - | llvm-dwarfdump - | FileCheck %s
+; RUN: %llc_dwarf -filetype=obj  %s -o - | llvm-dwarfdump - | FileCheck %s
 ; CHECK:       DW_TAG_string_type
 ; CHECK:                          DW_AT_name  ("character(*)!2")
 ; CHECK-NEXT:                     DW_AT_string_length