linux: bump the default version from 3.10.0 to 3.17.0

This is required for Qt5, as found in Ubuntu Focal. The library contains
the minimum kernel version encoded in an ELF note; this makes rtld ignore
it altogether, with a confusing error message. Without it, things fail
like this:

$ konsole: error while loading shared libraries: libQt5Core.so.5: cannot
open shared object file: No such file or directory

For reference, the Qt kernel version requirements can be found at:
https://github.com/qt/qtbase/blob/dev/src/corelib/global/minimum-linux_p.h

Sponsored by: The FreeBSD Foundation
Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D28105