chore: asmc: use designated initializers in macros

This code cleanup makes it easier for human readers to understand what each
of the fields actually represents, as well as makes it easier to modify
what the macros actually do under the covers, without introducing
potential human errors.

No functional change intended.

MFC after: 1 week

(cherry picked from commit ef1cde51032d395492caae302fedf183c58a482b)