Fix sys.kern.coredump_phnum_test.coredump_phnum on i386

The zero-padding when printing out the Size field is on 32-bit architectures is
5, not 15. Adjust the regular expression to work with both the 32-bit and
64-bit case.

MFC after: 1 week
Reviewed by: lwhsu, markj
Approved by: emaste (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D20005