Ensure the I-Cache is correctly handled in arm64_icache_sync_range

The cache_handle_range macro to handle the arm64 instruction and data
cache operations would return when it was complete. This causes problems
for arm64_icache_sync_range and arm64_icache_sync_range_checked as they
assume they can execute the i-cache handling instruction after it has been
called.

Fix this by making this assumption correct.

While here add missing instruction barriers and adjust the style to
match the rest of the assembly.

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18838