mail/rspamd: fix plist on !amd64

Plist token %%HYPERSCAN%% is not substituted if the corresponding
option doesn't exist. Manually add HYPERSCAN="@comment" when the
option is not defined.

Tested on armv7. The arm64 build is still broken due to an outdated
bundled simdutf, which should probably be unbundled.

Approved by: portmgr (build fix blanket)
MFH: 2026Q2

(cherry picked from commit c6b24b28a177f3bb93463f3987372938b104343f)