Make the arm port build cleanly with Clang.

This fixes the following two warnings, so that "make all" builds cleanly
with Clang:

  src/arm/macro-assembler-arm.h:1410:7: error: private field
  'instructions_' is not used
        [-Werror,-Wunused-private-field]
          int instructions_;  // Number of instructions of the expected patch size.
              ^

  src/arm/simulator-arm.cc:402:20: error: variable 'words' is used uninitialized whenever 'if'
        condition is false [-Werror,-Wsometimes-uninitialized]
          } else if (argc == next_arg + 1) {
                     ^~~~~~~~~~~~~~~~~~~~
  ../src/arm/simulator-arm.cc:407:21: note: uninitialized use occurs here
          end = cur + words;
                      ^~~~~
  ../src/arm/simulator-arm.cc:402:16: note: remove the 'if' if its condition is always true
          } else if (argc == next_arg + 1) {
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~

Review URL: https://chromiumcodereview.appspot.com/12087131

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13583 ce2b1a6d-e550-0410-aec6-3dcde31c8c00