MFC r275616:
Thread waiting for the vfork(2)-ed child to exec or exit, must allow
for the suspension.

MFC r275683 (by andreast):
Fix build for powerpc(32|64) kernels.

MFC r275686 (by andreast):
Fix kernel build for booke.

r275639 (by andrew) is not merged, since arm/arm/syscall.c is not
present on the stable/10 branch, and arm/arm/trap.c already includes
sys/kernel.h.