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>

Makefile

@@ -1826,10 +1826,15 @@ rusttest: prepare
 	$(Q)$(MAKE) $(build)=rust $@
 
 # Formatting targets
+#
+# Generated files as well as vendored crates are skipped.
 PHONY += rustfmt rustfmtcheck
 
 rustfmt:
 	$(Q)find $(srctree) $(RCS_FIND_IGNORE) \
+		\( \
+			-path $(srctree)/rust/proc-macro2 \
+		\) -prune -o \
 		-type f -a -name '*.rs' -a ! -name '*generated*' -print \
 		| xargs $(RUSTFMT) $(rustfmt_flags)