Work around GCC 6.3.1 bug

Work around GCC 6.3.1 bug

This commit works around a GCC 6.3.1 bug several people are hitting:

  https://sourceware.org/ml/gdb-patches/2017-09/msg00270.html
  https://sourceware.org/ml/gdb-patches/2017-10/msg00418.html

It manifests like this:

  ../../../binutils-gdb/gdb/probe.c:68:12: error: types may not be defined in a for-range-declaration [-Werror]
	 for (struct probe *probe : probes)
	      ^~~~~~

Fix it by renaming the range-for named variables to something different
from their type's name.

gdb/ChangeLog:
2017-10-16  Pedro Alves  <palves@redhat.com>

	* elfread.c (probe_key_free): Rename range-for variable.
	* probe.c (parse_probes_in_pspace, find_probes_in_objfile)
	(find_probe_by_pc, collect_probes): Rename range-for variable.