Use GCC 10 on Amazon Linux 2.

[dumol]

While working on #58, I gave GCC 10 packages on Amazon Linux a quick try. Had to reverse because of a Python test failure related to C++ modules. Starting diff (reverted): ced2651

Might be worth the trouble trying harder to build the generic glibc Linux packages with a newer GCC while retaining backward compatibility.

This is somehow related to #56, where using GCC 7 on Amazon Linux 1 (2018.03) would have brought RHEL 7 compatibility.