elfctl: Fix type errors.

Target value for val has uint32_t type, not uint, adjust used constant.
Change val type to unsigned so that left and right sides of comparision
operator do not expose different signed types of same range [*].

Switch to unsigned long long and strtoll(3) so that 0x80000000 is
accepted by conversion function [**].

Reported by: kargl [*]
Noted by: emaste [**]
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28301