genoffset.sh: Use 10 X's instead of 5 for pick mkdtemp implementations

Linux fails to build now because the mkdtemp in the bootstrapped
environment wants 6 or more X's. Use 10 out of an abundance of caution.

Sponsored by: Netflix
Reviewed by: arichards
Differential Revision: https://reviews.freebsd.org/D31863

(cherry picked from commit ecfbb2e30241ee460617ad4e0c0be16d9930945a)