[debug][panic] remove the wrapper panic #define and read the caller inside panic itself

Was always this way for some reason, which would tend to print the
calling routine that called the routine that paniced, which was of
dubious use. Simplify the panic logic and just call it as a standard
varargs routine.

top/include/lk/debug.h

@@ -51,8 +51,7 @@ static inline void hexdump8(const void *ptr, size_t len) {
 #define dprintf(level, x...) do { if ((level) <= LK_DEBUGLEVEL) { printf(x); } } while (0)
 
 /* systemwide halts */
-void _panic(void *caller, const char *fmt, ...) __PRINTFLIKE(2, 3) __NO_RETURN;
-#define panic(x...) _panic(__GET_CALLER(), x)
+void panic(const char *fmt, ...) __PRINTFLIKE(1, 2) __NO_RETURN;
 
 #define PANIC_UNIMPLEMENTED panic("%s unimplemented\n", __PRETTY_FUNCTION__)