Implement more security stuff

- Support deterministic stacks on OpenBSD
- Support OpenBSD system call origin verification
- Fix overrun by one in chibicc string token allocator
- Get all chibicc tests passing under Address Sanitizer

libc/macros.internal.inc

@@ -33,6 +33,9 @@
 .macro	.text.exit
 	.section .text.exit,"ax",@progbits
 .endm
+.macro	.text.syscall
+	.section .text.syscall,"ax",@progbits
+.endm
 .macro	.firstclass
 	.section .text.hot,"ax",@progbits
 .endm