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/getcontext_test.c

@@ -22,6 +22,8 @@
 #include "libc/testlib/ezbench.h"
 #include "libc/testlib/testlib.h"
 
+#ifdef __x86_64__
+
 int x;
 bool ok1;
 bool ok2;
@@ -62,3 +64,5 @@ void SetGetContext(void) {
 BENCH(getcontext, bench) {
   EZBENCH2("get/setcontext", donothing, SetGetContext());
 }
+
+#endif /* __x86_64__ */