pkg: beam: fix potential int overflow due to vague type

This patch fixes the beam package so that the int type used isn't vague
(i.e. it is consistently int64 on all platforms). This is a patch which
makes it easier to port docker to ARM (as this is the only error when
trying to compile docker on my ARMv7 machine).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)