Make include/malloc.h usable again.

Lot of third-party Linux code uses #include <malloc.h>, expecting to
find the malloc extensions there. Instead of trying to fight them,
accept that attempt to deprecate the header causes more troubles than
solves potential portability issues, and provide our jemalloc
extensions.

PR: 155429
Reviewed by: imp, jhibbits, dab, hselasky, philip, emaste, jilles
Exp-run by: antoine (PR 245366)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D24297