Makes the ARM CPU type a choice between CPU_CORTEX_M3 and CPU_CORTEX_M4
(similar to the choice of processor for x86). This completes the breaking
of the forced selection of CPU_CORTEX_M3 when CPU_CORTEX_M4 chosen, thereby
making CPU_CORTEX_M3_M4 the true umbrella option for the Cortex M3/M4
processors.
Change-Id: I1d1f39352d85a5848b13102ceb8365c7dede0959
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>