bpo-27987 for v3.5.10 and v3.6.15: align by 16bytes on 64bit platforms

[chaimleib]

Description

This resolves an error encountered on x86_64 for python versions 3.5.10 and 3.6.15 where ./python -c 'import ctypes' causes an immediate segfault. Also, when pyenv install runs ./python -E -m ensurepip --upgrade --root=/, this imports ctypes and also causes a segfault.

The segfault causes pyenv install 3.5.10 and the same for 3.6.15 to fail and not install.

This bug was resolved in bpo-27987 for the v2.7 branch and v3.7 branch and later. (Note that in the conversation, although one of the patches was reverted, the revert was reverted at the end.) This PR backports those patches for 3.5.10 and 3.6.15, which were left unfixed. These patches would likely not be accepted upstream because 3.5 and 3.6 are EOL.

Despite these versions being EOL, I was interested for old times sake. Also, the test harness for chaimleib/intervaltree lists these versions, among others, as test targets.

Tests

set -e
for v in 3.5.10 3.6.15; do
  pyenv install $v
  pyenv global $v
  python --version
  python -c 'import ctypes'
done

[native-api]

This looks like a solution for some old issues -- #2141 and #2046!

[native-api]

The errors are because 3.5.10 is incompatible with MacOS 11+ (3.6.15 has downstream patches that make it compatible). Going to fix that? We're fine either way since this is not an additional breakage.

[native-api]

Well done! And thank you!