eventfd is not available on FreeBSD 12

[Noratrieb]

20c13d4 added support for eventfd on FreeBSD. FreeBSD has eventfd in FreeBSD 13, but not in FreeBSD 12. FreeBSD 12 is still supported upstream and therefore rustc builds its distribution builds using FreeBSD 12, which fails because of the rustix change. rustix::backend::syscalls::eventfd gets linked into librustc_driver.so despite not being used (I'm not sure why, maybe something weird around dylibs?) and then it fails to link with the main binary because of the undefined symbol.

See rust-lang/rust#113046 (comment)

[sunfishcode]

This is now fixed in #718 and #717, and released in 0.38.2 and 0.37.22. I've also now re-enabled FreeBSD 12 testing, and fixed a few other additional minor issues that turned up.