Check tracefile is generated by binary execution

In gdb.trace/tfile.exp, we execute binary to generate tracefile,

  remote_exec target "$binfile"

however, this fails on bare metal target.  This patch is to
handle binary execution failure by running binary in GDB.
The binary will do some io operation to generate tracefile, so
we need a check 'target_info exists gdb,nofileio'.

This patch is to check whether tracefile is generated.  tfile.exp can
be skipped if generation is failed, while test_tfind_tfile in
mi-traceframe-changed.exp is skipped if generated failed.  The rest of
the mi-traceframe-changed.exp can still be executed, because on some
bare metal targets, the remote stub supports tracepoint but doesn't
support fileio.

gdb/testsuite:

2014-04-22  Yao Qi  <yao@codesourcery.com>

	* lib/trace-support.exp (generate_tracefile): New procedure.
	* gdb.trace/tfile.exp: Skip the test if generate_tracefile
	return 0.
	* gdb.trace/mi-traceframe-changed.exp: Invoke test_tfind_tfile
	if generate_tracefile returns 1.