arch/x86: CPUs expose an unimplemented feature

Instruction cache related functions (see include/zephyr/arch/cache.h)
are not implemented in x86, thus let's not set CONFIG_CPU_HAS_ICACHE
which may end up in build failure if one sets CONFIG_ICACHE.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>