Add checks that heap size is a multiple of 4 KiB

[axel-h]

With from seL4/seL4_libs#63 (related also #109) the heap size must be a multiple of 4 KiB. We should try check this at compile time to avoid runtime errors like seL4/camkes-vm#28 happening then. Also update the CMake comment at CAMKES_DEFAULT_HEAP_SIZE to mention at least, that certain alignment might be a runtime library requirement.
If we break compilation, we also force the user to make a proper choice - might be better any trying fix the value. The memory management lib's startup code should also do a runtime check at startup. It might reduce the available heap in debug and release builds then.