Cirrus-CI: add a manual amd64-gcc9 build and smoketest job

Allow users to test changes and find GCC-specific issues using Cirrus-CI
against their own GitHub forks.

Reviewed by: lwhsu, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34289

(cherry picked from commit 75e44873df2c86b9e15544b496af7040f9f6e0d2)