buildroot

jemalloc: add a check for valgrind being configured as well

jemalloc can be compiled with valgrind support being enabled
to produce proper output, so when the valgrind package has
been configured configure jemalloc appropriately.

In addition, make sure to disable valgrind support unconditionally in
the host variant of jemalloc.

Signed-off-by: Charles Hardin <ckhardin@exablox.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>