rust: proc-macro2: enable support in kbuild

With all the new files in place and ready from the new crate, enable
the support for it in the build system.

`proc_macro_byte_character` and `proc_macro_c_str_literals` were
stabilized in Rust 1.79.0 [1] and were implemented earlier than our
minimum Rust version (1.78) [2][3]. Thus just enable them instead of using
the `cfg` that `proc-macro2` uses to emulate them in older compilers.

In addition, skip formatting for this vendored crate and take the chance
to add a comment mentioning this.

Link: https://github.com/rust-lang/rust/pull/123431 [1]
Link: https://github.com/rust-lang/rust/pull/112711 [2]
Link: https://github.com/rust-lang/rust/pull/119651 [3]
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-11-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/generate_rust_analyzer.py

@@ -86,6 +86,13 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs, core_edit
         [],
     )
 
+    append_crate(
+        "proc_macro2",
+        srctree / "rust" / "proc-macro2" / "lib.rs",
+        ["core", "alloc", "std", "proc_macro"],
+        cfg=crates_cfgs["proc_macro2"],
+    )
+
     append_crate(
         "macros",
         srctree / "rust" / "macros" / "lib.rs",