Introduce cosmocc flags -mdbg -mtiny -moptlinux

The cosmocc.zip toolchain will now include four builds of the libcosmo.a
runtime libraries. You can pass the -mdbg flag if you want to debug your
cosmopolitan runtime. You can pass the -moptlinux flag if you don't want
windows code lurking in your binary. See tool/cosmocc/README.md for more
details on how these flags may be used and their important implications.

libc/runtime/clone.c

@@ -589,7 +589,9 @@ int sys_clone_linux(int flags,         // rdi
 
 static int LinuxThreadEntry(void *arg, int tid) {
   struct LinuxCloneArgs *wt = arg;
+#if defined(__x86_64__)
   sys_set_tls(ARCH_SET_GS, wt->tls);
+#endif
   return wt->func(wt->arg, tid);
 }