Revision 1.25 had the ATKBD_DFLT_KEYMAP option turned on and then off:

: # Options for atkbd:
: options ATKBD_DFLT_KEYMAP # specify the built-in keymap
: makeoptions ATKBD_DFLT_KEYMAP=jp.106
[...]
: nooption ATKBD_DFLT_KEYMAP
: nomakeoption ATKBD_DFLT_KEYMAP

(Previously the option was inherited from MI NOTES.) So my tool in
rev. 1.26 reduced this to removing all "ATKBD_DFLT_KEYMAP" lines,
leaving the option effectively disabled as it was before, but since
it's actually supported on sparc64, turn it on now.