Address a code generation bug that could allow the ARM stack protector
to be bypassed.

This change merges upstream r366369, r366371, and r267068 (minus some
test improvements).

Also:

• Address bugs breaking the build with all options disabled. [0]
• Pin the python version to 3.6 rather than 2.7.

PR: 239503 [0]
Security: https://kb.cert.org/vuls/id/129209