cross-build: Add comment missing from 9e5b0d9eac5b

Whilst the commit message documented some of the details, I had intended
to include this comment in the actual header, but failed to amend the
commit properly.

Fixes: 9e5b0d9eac5b ("cross-build: Fix bmake bootstrap with glibc 2.34")
MFC after: 1 week

(cherry picked from commit aee99ab4fe38bdc1dd1be88755d99ff4ee78ec09)