[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.boot.yaml

@@ -0,0 +1,26 @@
+---
+ld: clang++
+
+ccflags: [
+  "-nostdlib",
+  "-nodefaultlibs",
+  "-fno-builtin",
+
+  "-I${source_root}/external",
+  "--target=x86_64-unknown-windows",
+  "-ffreestanding",
+  "-mno-red-zone",
+  "-fshort-wchar",
+  "-fno-omit-frame-pointer",
+]
+
+cxxflags: [ "-fno-exceptions", "-fno-rtti" ]
+
+ldflags: [
+  "--target=x86_64-unknown-windows",
+  "-nostdlib",
+  "-Wl,-entry:efi_main",
+  "-Wl,-subsystem:efi_application",
+  "-fuse-ld=lld-link",
+]
+