Pull ARMv7-M code back in

This adds basic breakpoint/watchpoint support.

Changes include:
* Moved context state into scatter gather structure instead of
  mriCortexMState structure instead of the straight up array used
  before. Update armv7-m.c to make use of this new context format.
* Added fields to mriCortexMState to hold the value of the SP pointed
  to in the context scatter gather structure. Also added fields to
  track the fault cause such as exceptionNumber, DFSR, HFSR, etc. since
  for ThreadMRI it is better to capture these earlier in the exception
  handler before handing off to the main debugger thread as was done
  before in armv7-m.c  Not a bad idea to do it earlier in the case of
  handler mode MRI either.
* I will still need to do some work to get all of this working in the
  original MRI, handler mode code base.
* Handler mode MRI will still include system registers like MSP, PSP,
  etc. in the context while ThreadMRI will not.
* In mriCortexMInit(), skip the setting of SVCall, SysTick, and PendSV
  priorities for ThreadMRI. ThreadMRI also sets DebugMon to the lowest
  priority while handler mode MRI sets it to the highest priority.
* I updated code to use existing Platform_Get/SetProgramCounter() to
  manipulate the value of the PC register than grabbing it directly
  from the context which is now a bit more involved as it has to go
  through the scatter gather data structure.
* I haven't figured out how to implement single stepping in ThreadMRI
  so I have disabled it for now. This will be one of my next projects.
* Renamed global symbols to use "mri" prefix instead of "__mri" like I
  had already done in the other modules.

Author: adamgreen

libraries/ThreadMRI/examples/ThreadMRI/ThreadMRI.ino

@@ -13,7 +13,8 @@ void setup() {
     // UNDONE: Using Serial1 for now to unblock my progress.
     Serial1.begin(115200);
     g_debugger.debugException();
-    testContext();
+    //testContext();
+    __debugbreak();
 }
 
 void loop() {