x86/CET: Update vfork to prevent child return

x86/CET: Update vfork to prevent child return

Child of vfork should either call _exit or one of the exec family of
functions.  But normally there is nothing to prevent child of vfork from
return of the vfork-calling function.  Simpilfy x86 vfork when shadow
stack is in use to introduce mismatched shadow stack in child of vfork
to trigger SIGSEGV when the child returns from the function in which
vfork was called.