sort: Only build FreeBSD-specific ALTMON_x stuff when ATLMON_1 is defined

On MacOS, we bootstrap sort. Since ALTMON_* are not defined there, the
build blows up. Since we don't need this feature for the FreeBSD build
process, and since we won't use it unless we actually install the NL
files that have this data in it, just #ifdef it out for now. In the
extremely unlikely event that the FreeBSD bootstrap/build process grows
this dependency, we can evaluate the best solution then (which most
likely is going to be not depend on the local's month names).

Fixes: 3d44dce90a69 (MacOS builds and github CI)
Sponsored by: Netflix
Reviewed by: jrtc27, jlduran@gmail.com, markj
Differential Revision: https://reviews.freebsd.org/D42868