gdb: Workaround stringop-overread warning in debuginfod-support.c on s390x

For some reason g++ 11.2.1 on s390x produces a spurious warning for
stringop-overread in debuginfod_is_enabled for url_view. Add a new
DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD macro to suppress this warning.

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD): New
	macro.

gdb/ChangeLog:

	* debuginfod-support.c (debuginfod_is_enabled): Use
	DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD on s390x.

gdb/debuginfod-support.c

@@ -17,6 +17,7 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "defs.h"
+#include "diagnostics.h"
 #include <errno.h>
 #include "gdbsupport/scoped_fd.h"
 #include "debuginfod-support.h"
@@ -192,7 +193,17 @@ debuginfod_is_enabled ()
 	  if (off == gdb::string_view::npos)
 	    break;
 	  url_view = url_view.substr (off);
+#if defined (__s390x__)
+	  /* g++ 11.2.1 on s390x seems convinced url_view might be of
+	     SIZE_MAX length.  And so complains because the length of
+	     an array can only be PTRDIFF_MAX.  */
+	  DIAGNOSTIC_PUSH
+	  DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD
+#endif
 	  off = url_view.find_first_of (' ');
+#if defined (__s390x__)
+	  DIAGNOSTIC_POP
+#endif
 	  gdb_printf
 	    (_("  <%ps>\n"),
 	     styled_string (file_name_style.style (),

include/diagnostics.h

@@ -76,6 +76,9 @@
 # define DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION \
   DIAGNOSTIC_IGNORE ("-Wstringop-truncation")
 
+# define DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD \
+  DIAGNOSTIC_IGNORE ("-Wstringop-overread")
+
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL \
   DIAGNOSTIC_IGNORE ("-Wformat-nonliteral")
 
@@ -108,6 +111,10 @@
 # define DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION
 #endif
 
+#ifndef DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD
+# define DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD
+#endif
+
 #ifndef DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL
 #endif