sysutils/nix: fix build on armv7 and powerpc*
No libatomic on FreeBSD.
Also move manuals to PORTDOCS, the file names seem to be somewhat
architecture-dependent.
Approved by: portmgr (build fix blanket)
MFH: 2026Q1
(cherry picked from commit e2a13bf3e130067e79800864154bc92b37af1191)