build-llvm: Print a warning if we get more than one catch-all option

In this case it's usually an option that wasn't recognized
(although the misrecognized option may have been an earlier
option and the current one may be the intended prefix).

build-llvm.sh

@@ -92,6 +92,10 @@ while [ $# -gt 0 ]; do
         BUILDDIR="$BUILDDIR-pgo"
         ;;
     *)
+        if [ -n "$PREFIX" ]; then
+            echo Unrecognized parameter $1
+            exit 1
+        fi
         PREFIX="$1"
         ;;
     esac