malloc: Simplify static malloc interposition [BZ #20432]

malloc: Simplify static malloc interposition [BZ #20432]

Existing interposed mallocs do not define the glibc-internal
fork callbacks (and they should not), so statically interposed
mallocs lead to link failures because the strong reference from
fork pulls in glibc's malloc, resulting in multiple definitions
of malloc-related symbols.

(cherry picked from commit ef4f97648dc95849e417dd3e6328165de4c22185)

Conflicts:
	nptl/tst-tls3-malloc.c