Fix breakages in Linux-only build modes

- compile.com now polyfills -march=native which gcc/clang removed
- Guarantee zero Windows code is linked into non-Windows binaries
- MODE=tinylinux binaries are now back to being as tiny as ~4kb
- Improve the runtime's stack allocation / alignment hack
- GitHub Actions now tests Linux modes for assurance

libc/runtime/enable_tls.c

@@ -222,7 +222,9 @@ textstartup void __enable_tls(void) {
 
 #ifdef __x86_64__
   // rewrite the executable tls opcodes in memory
-  __morph_tls();
+  if (IsWindows() || IsXnu()) {
+    __morph_tls();
+  }
 #endif
 
   // we are now allowed to use tls