basic/missing_syscall: add missing_fchmodat2()

basic/missing_syscall: add missing_fchmodat2()

Follow-up for 8b45281daa3a87b4b7a3248263cd0ba929d15596
and preparation for later commits.

Since libcs are more interested in the POSIX `fchmodat(3)`, they are
unlikely to provide a direct wrapper for this syscall. Thus, the headers
we examine to set `HAVE_*` are picked somewhat arbitrarily.

Also, hook up `try_fchmodat2()` in `test-seccomp.c`. (Also, correct that
function's prototype, despite the fact that mistake would not matter in
practice)

Co-authored-by: Mike Yuan <me@yhndnzj.com>