Bump Linux version to 3.2.0. Without it, binaries linked against
glibc 2.24 and up (eg Ubuntu 19.04) fail with "FATAL: kernel too old".

This alone is not enough to make newer binaries actually work;
fix/hack/workaround is pending review at https://reviews.freebsd.org/D20687.

Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20757