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

libc/nexgen32e/gclongjmp.S

@@ -31,6 +31,7 @@
 //	@threadsafe
 //	@noreturn
 _gclongjmp:
+#ifdef __x86_64__
 	push	%rbp
 	mov	%rsp,%rbp
 	.profilable
@@ -59,4 +60,9 @@ _gclongjmp:
 2:	pop	%rsi
 	pop	%rdi
 	jmp	0b
+#elif defined(__aarch64__)
+	b	longjmp
+#else
+#error "unsupported architecture"
+#endif /* __x86_64__ */
 	.endfn	_gclongjmp,globl