build: treewide: Set retain attribute

LLD 13 and GNU ld 2.37 support -z start-stop-gc which allows garbage
collection of C identifier name sections despite the __start_/__stop_
references. GNU ld before 2015-10 had the behavior as well. Simply set
the retain attribute so that GCC 11 (if configure-time binutils is 2.36
or newer)/Clang 13 will set the SHF_GNU_RETAIN section attribute to
prevent garbage collection.

Without the patch, there are linker errors with -z start-stop-gc
(LLD default) when -Wl,--gc-sections is used:

```
ld.lld: error: undefined symbol: __start___eap
>>> referenced by eap.c
>>>               src/eap.o:(eap_init)
```

The remain attribute will not be needed if the metadata sections are
referenced by code directly.

client/command.h

@@ -83,10 +83,14 @@ struct command_family_desc {
 } __attribute__((aligned(8)));
 
 #define COMMAND_FAMILY(name, init, exit)				\
+	_Pragma("GCC diagnostic push")					\
+	_Pragma("GCC diagnostic ignored \"-Wattributes\"")		\
 	static struct command_family_desc __command_family_ ## name	\
-		__attribute__((used, section("__command"), aligned(8))) = {\
+		__attribute__((used, retain,				\
+				section("__command"), aligned(8))) = {	\
 			#name, init, exit				\
 		};							\
+	_Pragma("GCC diagnostic pop")
 
 bool command_init(char **argv, int argc);
 void command_exit(void);

client/dbus-proxy.h

@@ -118,10 +118,14 @@ struct interface_type_desc {
 } __attribute__((aligned(8)));
 
 #define INTERFACE_TYPE(interface, init, exit)				\
+	_Pragma("GCC diagnostic push")					\
+	_Pragma("GCC diagnostic ignored \"-Wattributes\"")		\
 	static struct interface_type_desc __interface_type_ ## interface\
-		__attribute__((used, section("__interface"), aligned(8))) = {\
+		__attribute__((used, retain, section("__interface"),	\
+					aligned(8))) = {		\
 			#interface, init, exit				\
 		};							\
+	_Pragma("GCC diagnostic pop")
 
 bool dbus_proxy_init(void);
 bool dbus_proxy_exit(void);

src/eap-private.h

@@ -94,10 +94,14 @@ struct eap_method_desc {
 } __attribute__((aligned(8)));
 
 #define EAP_METHOD_BUILTIN(name, init, exit)				\
+	_Pragma("GCC diagnostic push")					\
+	_Pragma("GCC diagnostic ignored \"-Wattributes\"")		\
 	static struct eap_method_desc __eap_builtin_ ## name		\
-		__attribute__((used, section("__eap"), aligned(8))) = {	\
+		__attribute__((used, retain, section("__eap"),		\
+			       aligned(8))) = {				\
 			#name, init, exit				\
 		};							\
+	_Pragma("GCC diagnostic pop")
 
 int eap_register_method(struct eap_method *method);
 int eap_unregister_method(struct eap_method *method);

src/module.h

@@ -33,19 +33,25 @@ struct iwd_module_depends {
 };
 
 #define IWD_MODULE(name, init, exit)					\
+	_Pragma("GCC diagnostic push")					\
+	_Pragma("GCC diagnostic ignored \"-Wattributes\"")		\
 	static struct iwd_module_desc __iwd_module_ ## name		\
-		__attribute__((used, section("__iwd_module"), aligned(8))) = {\
+		__attribute__((used, retain, section("__iwd_module"),   \
+			       aligned(8))) = {				\
 			#name, init, exit				\
-		};
+		};							\
+	_Pragma("GCC diagnostic pop")
 
 #define IWD_MODULE_DEPENDS(name, dep)					\
-	static struct iwd_module_depends				\
-				__iwd_module__##name##_##dep		\
-		__attribute__((used, section("__iwd_module_dep"),       \
-					aligned(8))) = {		\
-			.self = #name,					\
-			.target = #dep,					\
-		};
+	_Pragma("GCC diagnostic push")                                  \
+	_Pragma("GCC diagnostic ignored \"-Wattributes\"")              \
+	static struct iwd_module_depends __iwd_module__##name##_##dep	\
+		__attribute__((used, retain,				\
+			section("__iwd_module_dep"), aligned(8))) = {	\
+				.self = #name,				\
+				.target = #dep,				\
+		};							\
+	_Pragma("GCC diagnostic pop")
 
 int iwd_modules_init(void);
 void iwd_modules_exit(void);