basic/missing-syscalls: only emit one warning about missing numbers

The ifdef pattern is the same for all syscalls, so most of the time, if one is
not defined, all others will too. So let's reduce the noise a bit and emit one
warning in case the support for the architecture is fully missing. (Current
template was copied over from before when we added numbers for each syscall by
hand and stopped making sense when we started generating the header from a
table that is expected to have all syscall numbers.)

(cherry picked from commit 94dfd85b)