Add __mriPlatform_EnteringDebuggerHook() and __mriPlatform_LeavingDebuggerHook() to MRI.

Upgrades to version 0.5 of MRI.

A debuggee can implement these "C" hook routines to be called when MRI enters and
leaves a debug exception to do cleanup such as turn off heaters so that they
don't remain on while sitting in the debugger.

NOTE: You can assume very little about the state of your program from these
      routines.  I recommend that they be written in low level C code.  It
      isn't even safe to assume that C++ global constructors have been run
      before these routines are called.

Example which just toggles a LED on while in debug exception:
extern "C" void __mriPlatform_EnteringDebuggerHook(void)
{
    LPC_GPIO1->FIODIR |= 1 << 18;
    LPC_GPIO1->FIOPIN |= 1 << 18;
}

extern "C" void __mriPlatform_LeavingDebuggerHook(void)
{
    LPC_GPIO1->FIODIR |= 1 << 18;
    LPC_GPIO1->FIOPIN &= ~(1 << 18);
}