tree-wide: use gcc attribute macros where appropriate

We have these macros already, hence use them.

src/basic/hashmap.c

@@ -276,7 +276,7 @@ static const struct hashmap_type_info hashmap_type_info[_HASHMAP_TYPE_MAX] = {
 };
 
 #if VALGRIND
-__attribute__((destructor)) static void cleanup_pools(void) {
+_destructor_ static void cleanup_pools(void) {
         _cleanup_free_ char *t = NULL;
         int r;
 

src/basic/process-util.c

@@ -1172,7 +1172,7 @@ void reset_cached_pid(void) {
  * headers. __register_atfork() is mostly equivalent to pthread_atfork(), but doesn't require us to link against
  * libpthread, as it is part of glibc anyway. */
 extern int __register_atfork(void (*prepare) (void), void (*parent) (void), void (*child) (void), void *dso_handle);
-extern void* __dso_handle __attribute__ ((__weak__));
+extern void* __dso_handle _weak_;
 
 pid_t getpid_cached(void) {
         static bool installed = false;

src/basic/static-destruct.h

@@ -32,8 +32,8 @@ typedef struct StaticDestructor {
 
 /* Beginning and end of our section listing the destructors. We define these as weak as we want this to work even if
  * there's not a single destructor is defined in which case the section will be missing. */
-extern const struct StaticDestructor __attribute__((__weak__)) __start_SYSTEMD_STATIC_DESTRUCT[];
-extern const struct StaticDestructor __attribute__((__weak__)) __stop_SYSTEMD_STATIC_DESTRUCT[];
+extern const struct StaticDestructor _weak_ __start_SYSTEMD_STATIC_DESTRUCT[];
+extern const struct StaticDestructor _weak_ __stop_SYSTEMD_STATIC_DESTRUCT[];
 
 /* The function to destroy everything. (Note that this must be static inline, as it's key that it remains in the same
  * linking unit as the variables we want to destroy. */

src/basic/util.h

@@ -173,7 +173,7 @@ static inline void _reset_errno_(int *saved_errno) {
 }
 
 #define PROTECT_ERRNO                                                   \
-        _cleanup_(_reset_errno_) __attribute__((__unused__)) int _saved_errno_ = errno
+        _cleanup_(_reset_errno_) _unused_ int _saved_errno_ = errno
 
 static inline int negative_errno(void) {
         /* This helper should be used to shut up gcc if you know 'errno' is