ARM normally just returns 0 when dividing by 0 with the
software and hw implementations, which is different from
what X86 does. So, for NaCl, we've modified LLVM to trap
by inserting explicit 0 checks.

Uses -mattr=hwdiv-arm attribute to decide if 32-bit
sdiv/udiv are supported.

Also lower the unreachable-inst to a trap-inst, since we
need a trap instruction for divide by 0 anyway.

Misc: fix switch test under MINIMAL=1, since ARM requires
allow_dump for filetype=asm.
Random clang-format changes...

TODO: check via cross tests

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

Review URL: https://codereview.chromium.org/1214693004.