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

test/libc/calls/signal_test.c

@@ -48,7 +48,7 @@ void OnSigTrap(int sig, struct siginfo *si, void *ctx) {
 
 void TrapBench(int n) {
   for (int i = 0; i < n; ++i) {
-    asm("int3");
+    __builtin_trap();
   }
 }
 
@@ -72,6 +72,8 @@ BENCH(signal, trapBenchSiginfo) {
   sigaction(SIGTRAP, &old, 0);
 }
 
+#ifdef __x86_64__
+
 void OnSigHlt(int sig, struct siginfo *si, void *vctx) {
   struct ucontext *ctx = vctx;
   ctx->uc_mcontext.rip += 1;
@@ -94,3 +96,5 @@ BENCH(signal, hltBenchSiginfo) {
   sigaction(SIGSEGV, old + 0, 0);
   sigaction(SIGBUS, old + 1, 0);
 }
+
+#endif /* __x86_64__ */