are known to have the vfork syscall.
* sysdeps/m68k/sysdep.h (JUMPTARGET): Undefine before defining it.
* sysdeps/unix/sysv/linux/kernel-features.h
(__ASSUME_MMAP2_SYSCALL, __ASSUME_TRUNCATE64_SYSCALL)
(__ASSUME_STAT64_SYSCALL, __ASSUME_FCNTL64)
(__ASSUME_VFORK_SYSCALL): Define for m68k kernels >= 2.4.12.