futex: change to 6 args

This syscall takes up to 6 arguments, so change it from syscall4 to
syscall6 so people can pass those extra args along.

Change-Id: Ia78458a35b43433ddadd2c1eebfbf33d830b4e35
Reviewed-on: https://chromium-review.googlesource.com/c/linux-syscall-support/+/1436125
Reviewed-by: Mark Seaborn <mseaborn@chromium.org>