Fix limits.h constants to have correct type on MIPS

Use correctly typed constants to avoid bogus errors like
https://lists.freebsd.org/pipermail/svn-src-all/2017-August/150400.html
(like x86 _limits.h).

Reported by: bde, asomers
Reviewed by: mjoras, tinderbox
Sponsored by: Dell EMC Isilon