Because LIBS is not used anymore, tcmalloc/jemalloc does
not work with binaries whose description is in Meson.
The fix is simply to move them to Meson too.

For consistency with other configure options, specifying
--enable-malloc-trim together with --enable-{tc,je}malloc
becomes a fatal error.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>