[gdb/testsuite] Fix gdb.gdb/index-file.exp with -g0

When building gdb with -g0 and running test-case gdb.gdb/index-file.exp, we
run into:
...
(gdb) save gdb-index index_1^M
Error while writing index for `xgdb': No debugging symbols^M
(gdb) FAIL: gdb.gdb/index-file.exp: create gdb-index file
...

Fix this by instead emitting an unsupported, and bailing out.

Tested on aarch64-linux.

gdb/testsuite/gdb.gdb/index-file.exp

@@ -47,8 +47,19 @@ if { $worker_threads eq "UNKNOWN" } {
 set dir1 [standard_output_file "index_1"]
 remote_exec host "mkdir -p ${dir1}"
 with_timeout_factor $timeout_factor {
-    gdb_test_no_output "save gdb-index $dir1" \
-	"create gdb-index file"
+    set ok 0
+    gdb_test_multiple "save gdb-index $dir1" "create gdb-index file" {
+	-re -wrap "Error while writing index for \[^\r\n\]*: No debugging symbols" {
+	    unsupported $gdb_test_name
+	}
+	-re -wrap "^" {
+	    pass $gdb_test_name
+	    set ok 1
+	}
+    }
+    if { ! $ok } {
+	return -1
+    }
 
     gdb_test_no_output "save gdb-index -dwarf-5 $dir1" \
 	"create dwarf-index files"