Replace JEMALLOC_ATTR with various different macros when it makes sense

Theses newly added macros will be used to implement the equivalent under
MSVC. Also, move the definitions to headers, where they make more sense,
and for some, are even more useful there (e.g. malloc).

src/mutex.c

@@ -48,8 +48,7 @@ pthread_create_once(void)
 	isthreaded = true;
 }
 
-JEMALLOC_ATTR(visibility("default"))
-int
+JEMALLOC_EXPORT int
 pthread_create(pthread_t *__restrict thread,
     const pthread_attr_t *__restrict attr, void *(*start_routine)(void *),
     void *__restrict arg)
@@ -72,6 +71,7 @@ int	_pthread_mutex_init_calloc_cb(pthread_mutex_t *mutex,
 bool
 malloc_mutex_init(malloc_mutex_t *mutex)
 {
+
 #ifdef _WIN32
 	if (!InitializeCriticalSectionAndSpinCount(&mutex->lock,
 	    _CRT_SPINCOUNT))
@@ -98,7 +98,6 @@ malloc_mutex_init(malloc_mutex_t *mutex)
 		return (true);
 	}
 	pthread_mutexattr_destroy(&attr);
-
 #endif
 	return (false);
 }