(links, but crashes if an exception is thrown).

explicitly add gcc's own headers for stdarg.h

The critical wq allows pushing to remote cores and cores never
steal from the critical wq.

if necessary

The give a ~15% improvement in forkexecbench and makes xv6 a bit more
comparable to Linux.

the exec-cleanup and wait-cleanup handlers run on the right core
(disabled by default).

forkexectree now shares relatively little with
steal_nonexec=0 and wq_steal_others=0.

rather than the core that initially called exec.