Add CI build jobs for all supported targets even if libc-test is not set up for them

[gnzlbg]

There are some targets that we "support" (e.g. most *BSDs except FreeBSD), but for which we don't have any build jobs. We should add build jobs for these that check that libc cross-compiles correctly for these targets, even if we don't run libc-test to check libc's ABI.

This should at least make sure that these targets have no trivial compilation errors, and would also allow us to run the normal lints for these (clippy, make sure that all traits are implemented for the types available in these targets, etc.).