libtool.m4: Use echo in archive_cmds and archive_expsym_cmds on OS/2

Fixes that building DLLs fails because $ECHO does not work when it's
set to "printf %s\n".
From https://github.com/komh/libtool-os2

test/configure

@@ -6103,3 +6103,4 @@ if test -n "$ac_unrecognized_opts" && test "$enable_option_checking" != no; then
 printf "%s\n" "$as_me: WARNING: unrecognized options: $ac_unrecognized_opts" >&2;}
 fi
 
+