Double-check target_cflags in cpython-unix/targets.yml

[geofft]

Our native builds all have

    - '-fvisibility=hidden'
    # Needed to prevent BOLT from crashing.
    - '-fdebug-default-version=4'

The cross builds don't have -fvisibility=hidden and I sort of suspect it's not needed - 924693c does stuff in build-cpython.sh to handle symbol visibility differently, which applies to all our builds.

We also upgraded BOLT recently and we should double-check what's going on with that second flag.