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/sysv/errfun.S

@@ -33,6 +33,7 @@ __errfun:
 	str	w19,[x0]
 	mov	x0,#-1
 	ldp	x19,x30,[sp],#16
+	ret
 #else
 #error "unsupported architecture"
 #endif