Add proc_macro library source

Although provided as part of the pre-built toolchain, we did not include
its source code as it was not built in Soong. This adds the source to
improve developer tooling when building against proc_macro.

Change-Id: I7f77540e964f3107c30de4f5fbae6af697497de2

Author: tweksteen

build.py

@@ -34,6 +34,7 @@
         "src/libcore",
         "src/libpanic_abort",
         "src/libpanic_unwind",
+        "src/libproc_macro",
         "src/libprofiler_builtins",
         "src/libstd",
         "src/libterm",