jemalloc: restore JEMALLOC_DEBUG when building WITHOUT_MALLOC_PRODUCTION

The default for MALLOC_PRODUCTION was switched to ON in 02611ef8ee9f. This
effectively reverts the jemalloc header change from bfd15705156b so that
the knob behaves exactly as it does on a -CURRENT; that is, we are
effectively doing:

WITH_MALLOC_PRODUCTION -> -DMALLOC_PRODUCTION (default for stable/* and on)
WITHOUT_MALLOC_PRODUCTION -> -DJEMALLOC_DEBUG (default for main)

This allows the knob to be used for debugging on stable branches, too, which
is believed to be the main reason one would want to twist it off.

(direct commit)