Always initialize thread local storage

We had previously not enabled TLS in MODE=tiny in order to keep the
smallest example programs (e.g. life.com) just 16kb in size. But it
was error prone doing that, so now we just always enable it because
this change uses hacks to ensure it won't increase life.com's size.

This change also fixes a bug on NetBSD, where signal handlers would
break thread local storage if SA_SIGINFO was being used. This looks
like it might be a bug in NetBSD, but it's got a simple workaround.

libc/runtime/arch_prctl.c

@@ -199,7 +199,7 @@ int arch_prctl(int code, int64_t addr) {
     case METAL:
       return arch_prctl_msr(code, addr);
     case FREEBSD:
-      /* claims support but it appears not */
+      // TODO(jart): this should use sysarch()
       return arch_prctl_freebsd(code, addr);
     case OPENBSD:
       return arch_prctl_openbsd(code, addr);