The Data member of MergeSyntheticSection, which is used by ICF to check
equality of section contents, is always empty. This can result in
incorrect and dangerous deduplication when using --icf=all and
--ignore-data-address-equality.
This simple fix populates the Data of MergeSyntheticSection with the
intended contents in finalizeContents and uses Data for writeTo.
Fixes bug PR36910.