imgact_elf: compute auxv buffer size instead of using magic value

The new buffer is somewhat larger, but there should be no functional
changes.

Reviewed By: kib, imp
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30821

(cherry picked from commit 06250515cff552217d14145e8932cb1a543a7e07)