runtime: unify GC interface

Make sure every to-be-implemented GC can use the same interface. As a
result, a 1MB chunk of RAM is allocated on Unix systems on init instead
of allocating on demand.

src/runtime/arch_amd64.go

@@ -6,3 +6,8 @@ const GOARCH = "amd64"
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 64
+
+// Align on word boundary.
+func align(ptr uintptr) uintptr {
+	return (ptr + 7) &^ 7
+}