arm64 pmap: Eliminate redundant icache synchronizationHEADmain

Creating an executable user-space mapping to write-back memory
synchronizes the icache with the page's contents, whether or not those
contents have changed since the previous synchronization.  Use the pmap
private page flag PGA_ICACHE_SYNCED to record that the icache has been
synchronized with a managed page's contents and that the page has no
writable mappings.  When the flag is set, the creation of another
executable mapping to the page can skip the synchronization.  The flag
is cleared when a writable mapping to the page is created, using a
single atomic operation that keeps PGA_WRITEABLE and PGA_ICACHE_SYNCED
from ever being simultaneously set, and when the page's last mapping is
destroyed.

Assisted-by:	Claude Code (Fable 5.1)
Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D59865

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