Subzero, MIPS32: Randomly insert NOP

Patch implements randomlyInsertNop for MIPS32. Separate test case
added because current test is based on function with vector
operands, which MIPS32 does not support yet.
run-pnacl-sz.py updated, to pass -z argument to objdump, so it does
not hide multiple nops (shows ... instead) between instruction.

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2340733003 .

Patch from Srdjan Obucina <Srdjan.Obucina@imgtec.com>.