[BUGFIX] RTE: Incorrect rendering of toolbar labels

This is a follow-up to https://review.typo3.org/#/c/35487/

Releases: master
Resolves: #63794
Change-Id: Id860722c2b4f5b0d4c9f2782e2f24ff0c3f86572
Reviewed-on: http://review.typo3.org/35524
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>