Reintroduce setup.py changes from #8280 erased by piper import #8617

[jtattermusch]

Fixes #8667.

For an unknown reason (am I missing something?), setup.py changes from #8280 have been erased by piper import #8617 (see #8667 (comment)).

These changes are actually necessary for correct build of aarch64 python wheels.
without them, the aarch64 wheels will have python extension .so files with wrong arch suffix x86_64-linux-gnu.so,
which results in failing to load the python C++ extension.
This is also basically what the broken tests reported by #8667 have detected.

This PR fixes the problem - as proven by a green adhoc run of the kokoro job (https://fusion2.corp.google.com/invocations/c1e5e71e-e779-48e7-8f0a-23b1d33a06b0/targets/protobuf%2Fgithub%2Fmaster%2Fubuntu%2Fpython_aarch64%2Fcontinuous/log)

One thing I haven't been able to explain is:

• why did the piper import erase the changes made to setup.py (was that intentional or perhaps a bad merge?)
• was there any other important changes that ended up being deleted?

[jtattermusch]

Test failures seem unrelated, I'm going to merge.