Treat libmri4sim/mocks as tests instead of src

I previously had the mocks building as src instead of tests so that I
could measure its code coverage but I don't really want the mocks to be
part of the production library.
Add breakOnStart and exit() semihost call support

mri4simRun() now takes a breakOnStart parameter which indicates whether
the first call to pinkySimRun() should be skipped and instead jump
directly into the debugger.  I can later provide a command line flag
which allows the setting of this flag so that user's can setup
breakpoints before their code even starts executing.

MRI already has support for various ARM Angel monitor calls
(breakpoints) because of their use by the mbed code base for things
like the LocalFileSystem.  I have added code to the mri4simRun()
routine which will notice when the exit() semihost request (R0 = 0x18
and BKPT #0xAB) is encountered and will exit back to the caller.  This
can be used as a clean way to shutdown the simulator from within the
program running under simulation.

I also added an emitMOVimmediate() helper routine to mri4simBaseTest.h
to make it easier to test execution of instructions like:
  mov r0, #0x18