arm64: prevent panic when using syscall mux + large arg call (mmap)

if the syscall muxes are used, up to two additional arguments
may be required. This means that the 8 required for mmap increases
up to 10 (for __syscall).

Sponsored by: Juniper Networks, Inc.
Approved by: so
Security: FreeBSD-EN-25:15.arm64

(cherry picked from commit 740b879c6ade531adebeba7cd2f261bbe650797f)
(cherry picked from commit 17d87881a363c160e7e8cdb252d0261214c1a50b)