[gdb/testsuite] Add test-case gold-gdb-index.exp

[gdb/testsuite] Add test-case gold-gdb-index.exp

There's a PR binutils/15646 - "gold-generated .gdb_index has duplicated
symbols that gdb-generated index doesn't", and gdb contains a workaround,
added in commit 8943b87476 "Work around gold/15646".

Add a test-case testing this workaround.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-26  Tom de Vries  <tdevries@suse.de>

	* gdb.base/gold-gdb-index-2.c: New test.
	* gdb.base/gold-gdb-index.c: New test.
	* gdb.base/gold-gdb-index.exp: New file.
	* gdb.base/gold-gdb-index.h: New test.