Clang appears to be missing an include path to find
bits/c++config.h so we were unable to compile the
unsandboxed c++ based cross tests and link against the
subzero unsandboxed ARM object files.

Work around this for now by finding and including the
missing path.

Turn on a few ARM cross tests that should be working
(mem_intrin and test_strengthreduce -- though the
strength-reduction isn't done for ARM). The test_bitmanip
still fails, because under Om1 we overflow the stack offset
and need to materialize that offset with a register first.

Update a few other references that still say x8632.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1232183002 .