remote-stdio-gdbserver: Pass "target" to remote_exec to delete file

remote-stdio-gdbserver: Pass "target" to remote_exec to delete file

As described here

  https://sourceware.org/bugzilla/show_bug.cgi?id=22841

there seems to be situations where the remote-stdio-gdbserver board
fails to delete the uploaded binary file.  Passing "target" fixes the
issue for Christian who reported the bug.

I did not experience this problem, but passing "target" to remote_exec
still works for me, so I'm fine with changing it.

Any objection?

gdb/testsuite/ChangeLog:

	PR gdb/22841
	* boards/remote-stdio-gdbserver.exp (${board}_file): Pass
	"target" to remote_exec.