soc: xtensa/intel_adsp/ace: fix _end location

The symbol _end is used to indicate the start of heap in
the common libc malloc code. On ACE, heap is in uncached
area. However, _end was in the cached area while end of
heap is in uncached area. This resulted in incorrect
calculation of heap size. So move _end into uncached
area so correct heap size can be calculated.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>