Add support for flushing the instruction cache of other processes. This is

Add support for flushing the instruction cache of other processes.  This is
needed for inserting and removing breakpoints through ptrace(2).

The approach here only works for CPUs that have a PIPT instruction cache
as we use aliased mappings to invalidate the instruction cache.  That doesn't
work on CPUs that have a virtually indexed instruction cache.

ok deraadt@, visa@