smp: add smp_rendezvous_cpu helper function

[aokblast]

Move commit back

[olce]

Indentation of continuation lines for arguments is not as per style(9) (4 spaces, not a 8-space TAB), but consistent with some other smp_rendezvous_*() functions, so I'm fine with that (and these could all be fixed in a subsequent commit).

[markj]

Looks ok, but I'd use int in the function signature.

sys/kern/subr_smp.c

628

The cpuid type is usually int (see ipi_cpu() for instance) or u_int, we should follow that convention instead of using a fixed-width type.