[XTENSA] Fix cache flush macro for D$/I$ aliasing/non-aliasing

For configurations that have aliasing in the data cache but
not in the instruction cache, we don't need to flush the
instruction cache. Thus, we didn't define the macros to
flush the instruction cache. Some cache-flush functions,
howerver, were using those macros.

Signed-off-by: Chris Zankel <chris@zankel.net>