headers/kernel: Define B_CURRENT_IMAGE_SYMBOL via __func__.

Previously, __haiku_init_before was a symbol that was included in
each (shared) object, and so it could be used to determine what
one we were in. Now, there are no such universal symbols that
are declared private to only the object, so we have to use
a different approach.

__func__ is defined as a const char* at the very beginning of
every function it's used in, set to a string of the function name
only, i.e., the arguments and return type are left off. So while
including that is perhaps not quite optimal, in practice this
definition is used extremely rarely (it was introduced by Haiku,
and it is used in only 2 applications at all that I could find --
WebKit and Canna.)

There really isn't any other way to get a pointer that we know
for certain is within the current object besides this one
without inserting one, but that really isn't merited just for this.
(__builtin_return_address() has problematic semantics wrt. inlining,
including linker-inlining.) So this will have to do.

headers/os/kernel/image.h

@@ -40,6 +40,11 @@ typedef struct {
 } image_info;
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 /* flags for clear_caches() */
 #define B_FLUSH_DCACHE				0x0001	/* data cache */
 #define B_FLUSH_ICACHE				0x0004	/* instruction cache */
@@ -63,15 +68,10 @@ typedef struct {
 #define B_APP_IMAGE_SYMBOL		((void*)(addr_t)0)
 	/* value that can be used instead of a pointer to a symbol in the program
 	   image. */
-#define B_CURRENT_IMAGE_SYMBOL	((void*)&__haiku_init_before)
+#define B_CURRENT_IMAGE_SYMBOL	((void*)&__func__)
 	/* pointer to a symbol in the callers image */
 
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-
 thread_id load_image(int32 argc, const char **argv, const char **environ);
 image_id load_add_on(const char *path);
 status_t unload_add_on(image_id image);