[gdb/build] Fix build for gcc < 11

[gdb/build] Fix build for gcc < 11

When building trunk on openSUSE Leap 15.3 with system gcc 7.5.0, I run into:
...
In file included from ../bfd/bfd.h:46:0,
                 from gdb/defs.h:37,
                 from gdb/debuginfod-support.c:19:
gdb/debuginfod-support.c: In function ‘bool debuginfod_is_enabled()’:
gdb/../include/diagnostics.h:42:3: error: unknown option after \
  ‘#pragma GCC diagnostic’ kind [-Werror=pragmas]
   _Pragma (DIAGNOSTIC_STRINGIFY (GCC diagnostic ignored option))
   ^
gdb/../include/diagnostics.h:80:3: note: in expansion of macro \
  ‘DIAGNOSTIC_IGNORE’
   DIAGNOSTIC_IGNORE ("-Wstringop-overread")
   ^~~~~~~~~~~~~~~~~
gdb/debuginfod-support.c:201:4: note: in expansion of macro \
  ‘DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD’
    DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD
    ^
...

The problem is that the warning -Wstringop-overread has been introduced for
gcc 11, and we can only tell gcc to ignore if it knows about it.

Fix this by guarding the DIAGNOSTIC_IGNORE_STRINGOP_OVERREAD definition in
diagnostics.c with '#if __GNUC__ >= 11'.

Tested on x86_64-linux, by completing a build.