[PATCH] arm64: alternative:flush cache with unpatched code

1. Boot. This happens once, and we end up putting *all* secondary cores into
   a tight loop anyway, so I don't see that the performance of
   __flush_icache_all is relevant

Native boot happens once. But, each VM that boots will slow down all of the other VM's on the system. A VM boot can happen thousands of times.

I really don't want to cause the whole system to hiccup for a millisecond or more when there are only a few cache lines that need to be invalidated.