Add autolabel and mentions entries to triagebot.toml

Author: Kobzol

triagebot.toml

@@ -252,6 +252,11 @@ trigger_files = [
     "compiler/rustc_attr_data_structures",
 ]
 
+[autolabel."A-stdarch"]
+trigger_files = [
+    "library/stdarch",
+]
+
 [autolabel."F-autodiff"]
 trigger_files = [
     "src/tools/enzyme",
@@ -870,6 +875,15 @@ gets adapted for the changes, if necessary.
 """
 cc = ["@rust-lang/miri", "@RalfJung", "@oli-obk", "@lcnr"]
 
+[mentions."library/stdarch"]
+message = """
+`stdarch` is developed in its own repository. If possible, consider \
+making this change to \
+[rust-lang/stdarch](https://github.com/rust-lang/stdarch) \
+instead.
+"""
+cc = ["@amanieu"]
+
 [mentions."library/portable-simd"]
 message = """
 Portable SIMD is developed in its own repository. If possible, consider \