auxv: make AT_BSDFLAGS unsigned

AT_BSDFLAGS shouldn't be sign extended on 64-bit systems so use a
uint32_t instead of an int.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D42365

(cherry picked from commit 326bf5089ca788d5ff1951eed7a9067281a2b65e)