xform: accomodate tokens like "10_1"
xform: accomodate tokens like "10_1"

Those show up in the Mac OS X 10.11 SDK in `availability`
annotations.

Closes PR 15154

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket