makesyscalls: don't make syscall.mk by default

[brooks]

Summary

We only want to produce syscall.mk for the main syscall table so default
to not producing it (send it to /dev/null) and add a syscalls.conf to
sys/kern to trigger the creation of sys/sys/syscall.mk. This eliminates
the need for entries in other syscalls.conf files and is a cleaner
pattern going forward.

[kevans]

It might not be a bad idea to later special-case /dev/null in a couple places so that we don't even bother writing temp files in the first place (i.e. shim it out in write_line and a couple other places to nops).

[imp]

Likely not worth special case...
I have a refactoring that makes this run in libc in a make friendly way...

[brooks]

FWIW, I hit this because I'm looking at generating parts of lib/libc/sys/Symbol.map on the way to a long overdue reimplementation of D14609. Fortunately it's quite a minor change that just touches all the places sysmk support does (unless I decide to generate more of the file which would require annotations).