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]