build_nexe.py: Don't use O0 translation on debug builds
build_nexe.py: Don't use O0 translation on debug builds

Post-merge PNaCl has a bug when translating O2-optimized bitcode with O0
translation for non-SSE processors, which is triggered by our newlib
biased-bitcode IRT debug build. (Fast-isel fails to translate sitofp
instructions of the form produced by O2 bitcode when it can't use SSE).

For now, don't use O0 translation in debug builds. This works around the
problem at the cost of some extra translation time for debug builds.
I will shortly fix or work around the bug that blocks switching to nacl-clang
for the IRT build, which will result in much faster builds for all the cases.

R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1156113008

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client