libprocstat: simplify auxv value conversion

Avoid a weird dance through the union and treat all 32-bit values as
unsigned integers. This avoids sign extension of flags and userspace
pointers.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42198

(cherry picked from commit 9735cc0e41825bb9e95d16433d381ffe4c190f38)