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

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

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=798b415

[Jim Stichnoth, 2014-06-12 18:27:29]

LGTM.

Minor concerns about whether raw transplant of command-line arguments into
output filenames will yield a valid filename, but really too minor to worry
about.

[jvoung (off chromium), 2014-06-12 19:54:43]

Committed patchset #2 manually as r798b415 (presubmit successful).