gcc: make the ld wrapper add paths via -rpath if there's already an -rpath arg

We'd prefer ld to always take LD_RUN_PATH into account, even if any
rpath is specified in the command-line. We could actually change the
wrapper this way but it would not be consistent with the case where
the SR is built with system gcc.

component/gcc/buildout.cfg

@@ -42,13 +42,11 @@ post-install =
   done
   cat <<EOF >ld
   #!/bin/sh -e
-  case \$#:\$1 in 0:|1:-*) ;; *)
-    if [ "\$LD_RUN_PATH" ]
-    then LD_RUN_PATH=\$LD_RUN_PATH$1
-    else set -- "\$@" $2
-    fi
+  for x; do case \$x in -rpath=*)
+    set -- "\$@" $2
+    ! break
     ;;
-  esac
+  esac; done && export LD_RUN_PATH=\$${LD_RUN_PATH:+\$LD_RUN_PATH:}$${1#:}
   exec ${binutils:location}/bin/ld "\$@"
   EOF
   chmod +x ld