[BUGFIX] Respect mountpoint variable in cObj menu runtime cache

The mountpoint variable must be respected in the key used for the
menu runtime cache. Without that part in the key all links generated
to a mount-point sub-page will link to the first mount target page
in the menu.

Resolves: #80970
Resolves: #62248
Releases: master, 8.7, 7.6
Change-Id: I8ccfebabd515d6da9f78388de51d24603e9fe532
Reviewed-on: https://review.typo3.org/57532


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>