Subzero: give crosstest .sz intermediate files names that depend on flags

Currently only the output has a unique name (supplied by the invocation),
but the intermediate files (.sz.s, .sz.o) can get overwritten (w/ different
optlevels, or targets).

Would be nice to keep them around for debugging. (bug may happen
for Om1 but not O2).

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/333713004