[gdb/testsuite] Compile index-cache.c with -Wl,--build-id

[gdb/testsuite] Compile index-cache.c with -Wl,--build-id

When testing gdb.base/index-cache.exp using a gcc build without
--enable-linker-build-id we get:
...
FAIL: gdb.base/index-cache.exp: \
  test_cache_enabled_miss: at least one file was created
FAIL: gdb.base/index-cache.exp: \
  test_cache_enabled_miss: couldn't get executable build id
FAIL: gdb.base/index-cache.exp: \
test_cache_enabled_hit: check index-cache stats
...

With "set debug index-cache on" we find:
...
(gdb) file index-cache
Reading symbols from index-cache...
index cache: objfile index-cache has no build id
...

The problem is that a build-id is required for the index-cache functionality.

Fix this by compiling index-cache.c with -Wl,--build-id.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-06-21  Tom de Vries  <tdevries@suse.de>

	* gdb.base/index-cache.exp: Add additional_flags=-Wl,--build-id.