Make considerably more progress on AARCH64

- Utilities like pledge.com now build
- kprintf() will no longer balk at 48-bit addresses
- There's a new aarch64-dbg build mode that should work
- gc() and defer() are mostly pacified; avoid using them on aarch64
- THIRD_PART_STB now has Arm Neon intrinsics for fast image handling

examples/auto-launch-gdb-on-crash.c

@@ -41,6 +41,6 @@
 
 int main(int argc, char *argv[]) {
   ShowCrashReports();
-  asm("int3"); /* cf. __die(), perror("msg"), abort(), exit(1), _Exit(1) */
+  __builtin_trap();
   return 0;
 }