Introduce cpu_icache_sync_range_checked(), that does the same thing as
cpu_icache_sync_range(), except that it sets pcb_onfault to catch any page
fault, as doing cache maintenance operations for non-mapped generates a
data abort, and use it in freebsd32_sysarch(), so that a userland program
attempting to sync the icache with unmapped addresses doesn't crash the
kernel.

Spotted out by: andrew