arch/arm: add support for noMMU

Some ARM cores do not have an MMU (eg. Cortex-Mx and Cortex-Rx series).
Thus, allow the user to configure out the MMU for ARM.

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }