Rename cortex-m architecture to armv7-m

ARMv7-M is the actual name of the architecture.  Cortex-M3 is a device
family which implements the ARMv7-M architecture.
Updated CMSIS headers from v3.00 to v3.20

I also removed utility functions like __mriGetIPSR(),
__mriSetBASEPRI(), etc from armv7-m_asm.S and used the equivalents now
available in CMSIS.

I should not that I added a __mriARMv7AsmInit() function in
armv7-m_asm.S to be called from the C initialization code.  This keeps
the ARMv7-M C code dependent on this assembly language module so that
the linker pulls in its object file.
Switch symbols from -gstabs+3 to -g3

I have made this switch sometime ago in most of my ARM projects but it
appears that MRI was missed.
Remove mri/stubs directory

These stubs were only required for older mbed LPC1768 platform specific
unit tests which no longer exist as they were too fragile.
Get gcov to work again

Pull fix for CppUTest and remove old coverage script.
Ignore gcov output files
Enable CPPUTEST_USE_VPATH for unit test build

This makes for a cleaner directory output when the sources being tested
are in directories like ../mri