triagebot: Label compiler-builtins T-libs

Changes to `compiler-builtins` don't currently get a `T-` label, but it
is mostly managed by libs. Add the autolabel here.

Author: tgross35

triagebot.toml

@@ -355,6 +355,7 @@ trigger_labels = [
 [autolabel."T-libs"]
 trigger_files = [
     "library/alloc",
+    "library/compiler-builtins",
     "library/core",
     "library/panic_abort",
     "library/panic_unwind",