Make marker probe support listing mode -L

Make marker probe support listing mode -L

This patch is to enable displaying the arguments of
marker probe for listing mode -L. The output is like,

$stap -L 'kernel.mark("*")'
kernel.mark("core_marker_format").format("name %s format %s") $arg1:string $arg2:string
kernel.mark("jbd2_checkpoint").format("dev %s need_checkpoint %d") $arg1:string
$arg2:long
kernel.mark("jbd2_end_commit").format("dev %s transaction %d head %d") $arg1:string $arg2:long $arg3:long
kernel.mark("jbd2_start_commit").format("dev %s transaction %d") $arg1:string $arg2:long

Note: It's also possible to figure out the arguments according to the format.

Signed-off-by: Wenji Huang <wenji.huang@oracle.com>