Implement pmap_sync_icache().

This invokes "fence" on the hart performing the write followed by an IPI
to execute "fence.i" on all harts.

This is required to support userland debuggers setting breakpoints in
user processes.

Reviewed by: br (earlier version), markj
Approved by: re (gjb)
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D17139