[gdb/testsuite] Fix index-cache.exp with cc-with-{gdb-index,debug-names}

[gdb/testsuite] Fix index-cache.exp with cc-with-{gdb-index,debug-names}

In gdb.base/index-cache.exp, handle the case that binfile contains either a
.gdb_index or .debug_names index section.

Tested on x86_64-linux with native, cc-with-gdb-index and cc-with-debug-names.

gdb/testsuite/ChangeLog:

2019-05-06  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (exec_has_index_section): New proc.
	* gdb.base/index-cache.exp: Handle case that binfile contains an index
	section.