In 38d1ac34ff SIGATOMIC_{MIN,MAX} were
defined in terms of LONG_{MIN,MAX}. Later, they were switched to
__LONG_{MIN,MAX} in 78fe75bc28 where an
include of machine/_limits.h was added. Switch to using fixed width
INT64_{MIN,MAX} and remove the header pollution.
No functional change.
Reviewed by: theraven, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D39196
eb232cffc9