jemalloc: Reduce diffs to 5.2.1 revert to using JEMALLOC_FALLTHROUGH

In the import of 5.2.1 (c5ad81420c495), JEMALLOC_FALLTHROUGH was changed
to break (judging by the tagged upstream sources). This changes it back
to upstream. No functional change: this is just a textual change (though
the fallthrough macro doesn't expand to break).

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