pbuilder: Update native to USERSPACE_ARCH

Corallary to https://coral-review.googlesource.com/c/build/+/12734 -- we need to
update all calls of the "native" toolchain to use the USERSPACE_ARCH variable to
match up with the naming change.

Change-Id: Ib01faa0900195cf84051ca729d0918cb7c929b9c
diff --git a/packages.mk b/packages.mk
index 96a0fcd..e594468 100644
--- a/packages.mk
+++ b/packages.mk
@@ -24,7 +24,7 @@
 $(eval $(call make-pbuilder-bsp-package-target,imx-vpu-hantro,imx-vpu-hantro,linux-imx,,binary))
 $(eval $(call make-pbuilder-bsp-package-target,imx-vpuwrap,imx-vpuwrap,imx-vpu-hantro,,binary))
 $(eval $(call make-pbuilder-bsp-package-target,imx-gstreamer,imx-gstreamer))
-$(eval $(call make-pbuilder-bsp-package-target,imx-gst-plugins-base,imx-gst-plugins-base,imx-gstreamer linux-imx,,,native))
+$(eval $(call make-pbuilder-bsp-package-target,imx-gst-plugins-base,imx-gst-plugins-base,imx-gstreamer linux-imx,,,$(USERSPACE_ARCH))))
 $(eval $(call make-pbuilder-bsp-package-target,gst-plugins-ugly1.0,packages/gst-plugins-ugly1.0,imx-gst-plugins-base))
 $(eval $(call make-pbuilder-bsp-package-target,imx-gst-plugins-good,imx-gst-plugins-good,imx-gst-plugins-base libdrm-imx))
 $(eval $(call make-pbuilder-bsp-package-target,imx-gst-plugins-bad,imx-gst-plugins-bad,\