Everywhere: Make some symbols `__attribute__((used))` for LTO

With these changes, the userland builds correctly with Clang's ThinLTO
enabled.

Kernel/Prekernel/init.cpp

@@ -16,7 +16,7 @@
 
 // Defined in the linker script
 extern size_t __stack_chk_guard;
-size_t __stack_chk_guard;
+size_t __stack_chk_guard __attribute__((used));
 extern "C" [[noreturn]] void __stack_chk_fail();
 
 extern "C" u8 start_of_prekernel_image[];

Kernel/init.cpp

@@ -69,7 +69,7 @@ extern ctor_func_t start_ctors[];
 extern ctor_func_t end_ctors[];
 
 extern size_t __stack_chk_guard;
-READONLY_AFTER_INIT size_t __stack_chk_guard;
+READONLY_AFTER_INIT size_t __stack_chk_guard __attribute__((used));
 
 extern "C" u8 start_of_safemem_text[];
 extern "C" u8 end_of_safemem_text[];

Userland/DynamicLoader/main.cpp

@@ -57,8 +57,8 @@ this helper program directly.
 extern "C" {
 
 // The compiler expects a previous declaration
-void _start(int, char**, char**);
-void _entry(int, char**, char**);
+void _start(int, char**, char**) __attribute__((used));
+void _entry(int, char**, char**) __attribute__((used));
 
 NAKED void _start(int, char**, char**)
 {

Userland/Libraries/LibC/crt0.cpp

@@ -19,8 +19,8 @@ extern size_t __stack_chk_guard;
 int main(int, char**, char**);
 
 // Tell the compiler that this may be called from somewhere else.
-int _entry(int argc, char** argv, char** env);
-void _start(int, char**, char**);
+int _entry(int argc, char** argv, char** env) __attribute__((used));
+void _start(int, char**, char**) __attribute__((used));
 
 NAKED void _start(int, char**, char**)
 {

Userland/Libraries/LibC/ssp.cpp

@@ -18,7 +18,7 @@
 extern "C" {
 
 extern size_t __stack_chk_guard;
-size_t __stack_chk_guard = (size_t)0xc6c7c8c9;
+__attribute__((used)) size_t __stack_chk_guard = (size_t)0xc6c7c8c9;
 
 __attribute__((noreturn)) void __stack_chk_fail()
 {