Correctly handle $syscall in process(PID_OR_PATH).syscall.return probes.

Correctly handle $syscall in process(PID_OR_PATH).syscall.return probes.
2009-04-21  David Smith  <dsmith@redhat.com>

	* tapsets.cxx (utrace_derived_probe::utrace_derived_probe): If
	during target-variable-expanding the probe, we added a new block
	or probe, add them.
	(utrace_var_expanding_visitor::visit_target_symbol_cached): New
	function to cache target variables in a generated syscall probe to
	use in syscall.return probes.
	(utrace_var_expanding_visitor::visit_target_symbol_context): In a
	syscall.return probe, you can't access $syscall.  So use
	visit_target_symbol_cached() to cache the value for use here.
	* testsuite/systemtap.base/utrace_syscall_args.stp: Test use of
	$syscall in syscall.return probes.