gcc-plugins: drop -std=gnu++11 to fix GCC 13 build

The latest GCC 13 snapshot (13.0.1 20230129) gives the following:
```
cc1: error: cannot load plugin ./scripts/gcc-plugins/randomize_layout_plugin.so
 :./scripts/gcc-plugins/randomize_layout_plugin.so: undefined symbol: tree_code_type
```

This ends up being because of https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=b0241ce6e37031
upstream in GCC which changes the visibility of some types used by the kernel's
plugin infrastructure like tree_code_type.

After discussion with the GCC folks, we found that the kernel needs to be building
plugins with the same flags used to build GCC - and GCC defaults to gnu++17
right now. The minimum GCC version needed to build the kernel is GCC 5.1
and GCC 5.1 already defaults to gnu++14 anyway, so just drop the flag, as
all GCCs that could be used to build GCC already default to an acceptable
version which was >= the version we forced via flags until now.

Bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108634
Signed-off-by: Sam James <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: thesamesam

scripts/gcc-plugins/Makefile

@@ -29,7 +29,7 @@ GCC_PLUGINS_DIR = $(shell $(CC) -print-file-name=plugin)
 plugin_cxxflags	= -Wp,-MMD,$(depfile) $(KBUILD_HOSTCXXFLAGS) -fPIC \
 		  -include $(srctree)/include/linux/compiler-version.h \
 		  -DPLUGIN_VERSION=$(call stringify,$(KERNELVERSION)) \
-		  -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++11 \
+		  -I $(GCC_PLUGINS_DIR)/include -I $(obj) \
 		  -fno-rtti -fno-exceptions -fasynchronous-unwind-tables \
 		  -ggdb -Wno-narrowing -Wno-unused-variable \
 		  -Wno-format-diag