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]

Add the initial implementation of the process kernel stack logic.  Not yet integrated into the main OS logic nor tested.

Rename everything associated with the dynamic process stack to ustack to make room in the name space for a kstack

Move static helper routines from arm_addrenv.c and may them global so that they can be shared both forthcoming stack address environment logic.

ARMv7-A: Modify up_fullcontextrestore() for CONFIG_BUILD_KERNEL.  It changed CPSR while in kernel.  That will crash is the new CPSR is user mode while executing in kernel space.  Fixed by adding a SYS_context_restore system call.  There is an alternative, simpler modification to up_fullcontextrestore() that could have been done:  It might have been possible to use the SPSR instead of the CPRSR and then do an exception return from up_fullcontextrestore().  That would be more efficient, but I never tried it.