amd64: reduce header pollution in _stdint.h

In 38d1ac34ff82bd2aeb308b52a65b686060e52873 SIGATOMIC_{MIN,MAX} were
defined in terms of LONG_{MIN,MAX}. Later, they were switched to
__LONG_{MIN,MAX} in 78fe75bc280264e7471b3069e148cae32e8ae211 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