[PATCH bpf-next] compiler-gcc: Ignore -Wmissing-prototypes warning for older GCC

[Yafang Shao <laoar.shao@gmail.com>]

The kernel supports a minimum GCC version of 5.1.0 for building. However,
the "__diag_ignore_all" directive only suppresses the
"-Wmissing-prototypes" warning for GCC versions >= 8.0.0. As a result, when
building the kernel with older GCC versions, warnings may be triggered. The
example below illustrates the warnings reported by the kernel test robot
using GCC 7.5.0:

  compiler: gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0
  All warnings (new ones prefixed by >>):

   kernel/bpf/helpers.c:1893:19: warning: no previous prototype for 'bpf_obj_new_impl' [-Wmissing-prototypes]
    __bpf_kfunc void *bpf_obj_new_impl(u64 local_type_id__k, void *meta__ign)
                      ^~~~~~~~~~~~~~~~
   kernel/bpf/helpers.c:1907:19: warning: no previous prototype for 'bpf_percpu_obj_new_impl' [-Wmissing-prototypes]
    __bpf_kfunc void *bpf_percpu_obj_new_impl(u64 local_type_id__k, void *meta__ign)
   [...]

To address this, we should also suppress the "-Wmissing-prototypes" warning
for older GCC versions. Since "#pragma GCC diagnostic push" is supported as
of GCC 4.6, it is acceptable to ignore these warnings for GCC >= 5.1.0.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311031651.A7crZEur-lkp@intel.com/
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/compiler-gcc.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 7af9e34..a5cfcad 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -131,14 +131,14 @@
 #define __diag_str(s)		__diag_str1(s)
 #define __diag(s)		_Pragma(__diag_str(GCC diagnostic s))
 
-#if GCC_VERSION >= 80000
-#define __diag_GCC_8(s)		__diag(s)
+#if GCC_VERSION >= 50100
+#define __diag_GCC_5(s)		__diag(s)
 #else
-#define __diag_GCC_8(s)
+#define __diag_GCC_5(s)
 #endif
 
 #define __diag_ignore_all(option, comment) \
-	__diag_GCC(8, ignore, option)
+	__diag_GCC(5, ignore, option)
 
 /*
  * Prior to 9.1, -Wno-alloc-size-larger-than (and therefore the "alloc_size"
-- 
1.8.3.1