[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.exe.yaml

@@ -0,0 +1,15 @@
+---
+
+ccflags: [
+  "-fpie"
+]
+
+ldflags: [
+  "-pie",
+  "--dynamic-linker", "/jsix/lib/ld.so",
+  "--push-state", "--as-needed", "-Bstatic", "-lc++", "-lc++abi", "-lunwind", "--pop-state",
+]
+
+libs: [
+  "${target_dir}/crt0.o",
+]