kernel: pin _kernel variable in case of paging

Exception handler(arch/x86/core/ia32/excstub.S) may access
_kernel variable, it will lead to failure when enabled paging,
so make this critical variable pinned.

Signed-off-by: Qipeng Zha <qipeng.zha@intel.com>

kernel/init.c

@@ -42,6 +42,7 @@ BUILD_ASSERT(CONFIG_MP_NUM_CPUS == CONFIG_MP_MAX_NUM_CPUS,
 	     "CONFIG_MP_NUM_CPUS and CONFIG_MP_MAX_NUM_CPUS need to be set the same");
 
 /* the only struct z_kernel instance */
+__pinned_bss
 struct z_kernel _kernel;
 
 /* init/main and idle threads */