ART: Do not use variable-length array.

Variable-length arrays are not part of the C++ standard.
While clang++ supports them as an extension, it does not
play well with address sanitizer in clang-r584948.

Test: SANITIZE_HOST=address ASAN_OPTIONS=detect_leaks=0 \
      testrunner.py -b --host --jit
Flag: EXEMPT BUGFIX
Change-Id: I29d77fa07546612d87e4f4d411837a7c32c8f5f5