xtensa: fix unneeded cache invalidation in arch_cohere_stacks()

During arch_cohere_stacks(), the used portion of the outgoing
thread is cache flushed, and then the unused portion of cache
invalidated. However, this results in the cache line at
the stack pointer being flushed and then invalidated due to
how sys_cache_data_*() operates. If we are swapping back to
the same thread (e.g. after handling interrupt), this cache
line will need to be retrieved again from main memory since
it has already been invalidated. This creates unnecessary
data move between cache and main memory. So create our own
version of cache flushing and invalidation routines just for
arch_cohere_stacks(). Bouns is that these work directly with
bounding addresses and skips the size calculation which should
save a little bit amount of execution time.

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