Fix early return in foreach_with_prefix

Fix early return in foreach_with_prefix

I noticed that an early return in a foreach_with_prefix block does not
cause the outer scope to return, like:

  foreach_with_prefix var {"foo" "bar"} {
     return
  }
  # Control continues here, but it should not.

The problem is that we're missing the usual "return -code" treatment.
This commit fixes it.

gdb/testsuite/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (foreach_with_prefix): Use "catch" and
	"return -code".