+ indicate which LLVM-MC versions are broken in the compiler help screen

compiler/msg/errore.msg

@@ -4035,8 +4035,8 @@ W*2Abinaryen_Assemble using GNU AS for wasm32 (wasm-as) (experimental)
 W*2Allvm-mc-10_Assemble using llvm-mc-10
 W*2Allvm-mc-11_Assemble using llvm-mc-11
 W*2Allvm-mc-12_Assemble using llvm-mc-12
-W*2Allvm-mc-13_Assemble using llvm-mc-13
-W*2Allvm-mc-14_Assemble using llvm-mc-14
+W*2Allvm-mc-13_Assemble using llvm-mc-13 (broken, due to bugs in LLVM)
+W*2Allvm-mc-14_Assemble using llvm-mc-14 (broken, due to bugs in LLVM)
 W*2Allvm-mc-15_Assemble using llvm-mc-15
 W*2Allvm-mc-16_Assemble using llvm-mc-16
 W*2Allvm-mc-17_Assemble using llvm-mc-17