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.

(cherry picked from commit 740b879c6ade531adebeba7cd2f261bbe650797f)