arm64: Add thread exit syscall in commpage.

Change-Id: Ia8e7e4626add623735fefefa1af151b7338adc35
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5265
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

arm64: Add stubs so kernel can at least link

Signed-off-by: Jaroslaw Pelczar <jarek@jpelczar.com>
Change-Id: I2476a6346c912c4aa0c26e4f3720ea2c2690b669
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1857
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>