[scripts] Make j6libc.py into the codegen package

j6libc.py was initially made for libc to generate stdint.h, but it
gained a few things that aren't libc-specific. Move it to a package and
split the int-types-specific code into codegen.int_types.

scripts/codegen/__init__.py

@@ -0,0 +1,16 @@
+import cog
+
+supported_architectures = {
+    "amd64": "__amd64__",
+}
+
+def arch_includes(header, root=""):
+    prefix = "if"
+    for arch, define in supported_architectures.items():
+        cog.outl(f"#{prefix} defined({define})")
+        cog.outl(f"#include <{root}arch/{arch}/{header}>")
+        prefix = "elif"
+    cog.outl("#else")
+    cog.outl('#error "Unsupported platform"')
+    cog.outl("#endif")
+