Fix vfork().  Now that arguments are kept on the stack, the way that arguments are passed from parent to child in vfork() must change.  This bug has always been present, but was not visible with the old strdup() way of passing arguments

Move common/up_signal_dispatch.c to armv6-m, armv7-m, and armv7-a.  The armv7-a version needs to be different to handle the case where we are dispatch kernel mode signals when running under a user mode group

If we are configured to use a kernel stack while in SYSCALL handling, then we need to switch back to the user stack to deliver a signal

Fix a typo in system call when fetching parameter from the stack:  regs[REG_PC]+4 is the address, not regs[REG_PC+4]