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/integral/c.inc

@@ -618,6 +618,7 @@ typedef uint64_t uintmax_t;
 #define textexit    _Section(".text.exit") noinstrument
 #define textreal    _Section(".text.real")
 #define textwindows _Section(".text.windows")
+#define textsyscall _Section(".text.syscall")
 #define antiquity   _Section(".text.antiquity")
 #else
 #define testonly