[build] first pass at multiarch support

Changing bonnibel to respect the --arch flag to configure. This requires
some reworking of modules, mostly in the addition of the ModuleList
class instead of just a dict of modules.

assets/build/amd64/target.user.shared.yaml

@@ -0,0 +1,7 @@
+---
+ccflags: [
+]
+
+ldflags: [
+  "-shared",
+]