Build jemalloc for host, too.

We build both static and dynamic libjemalloc and libjemalloc_jet for
the host. For the target, only the static libraries are built, just
as before.

Test: 'mmma external/jemalloc', verify that it builds the same target
libraries, verify it builds host libraries and they can be used to
build ckati.

Change-Id: I588b84fba7785d31dc5200a805a43b34bb0723d1