build: phase 0 — extract the build-support package

The shared user-binary recipe (freestanding target, root-shim wiring,
link-script and image-base settings) moves out of the root build into
build-support/, the package that is the single home for cross-cutting
build changes (docs/build-packages-plan.md). The root build's
addUserBinary/addThreadedUserBinary keep their signatures and delegate;
nothing else moves. Boot-image file list unchanged.

build-support/build.zig.zon

@@ -0,0 +1,8 @@
+.{
+    .name = .build_support,
+    .version = "0.0.0",
+    .fingerprint = 0xad91962994f4be41, // Changing this has security and trust implications.
+    .minimum_zig_version = "0.16.0",
+    .dependencies = .{},
+    .paths = .{""},
+}