Eliminate is_remote check in gdb.base/scope.exp

Eliminate is_remote check in gdb.base/scope.exp

This commit makes --target_board=native-gdbserver (and in principle
all other is_remote boards) pass all the same gdb.base/scope.exp tests
as native testing.

I first wrote the gdb.base/scope.exp change described in the ChangeLog
below and in the new comments in the patch, knowing that gdb_file_cmd
was the right thing to use here.

However, that revealed that the native-extended-gdbserver board should
be overriding gdb_file_cmd+gdb_reload instead of gdb_load, as is
hinted at by the comments on top of the default implementations in
testsuite/lib/gdb.exp, because otherwise a gdb_run_cmd after
gdb_file_cmd misses setting "set remote exec-file".  However, if we do
that and remove gdb_load, then we regress gdb.base/dbx.exp, so for now
keep the gdb_load override as well.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/scope.exp: Use build_executable + clean_restart +
	gdb_file_cmd instead of prepare_for_testing and no longer skip
	"before run" tests on is_remote target boards.  Update comments.
	* boards/native-extended-gdbserver.exp
	(extended_gdbserver_load_last_file): New, factored out from ...
	(gdb_load): ... this.  Move further below and add comment.
	(extended_gdbserver_gdb_file_cmd, gdb_file_cmd, gdb_reload): New.