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

third_party/xxhash/xxh_x86dispatch.c

@@ -82,10 +82,12 @@ extern "C" {
 #  define XXH_X86DISPATCH_ALLOW_AVX
 #endif
 
+#if 0 /* [jart] be quiet */
 #if defined(__AVX__) && !defined(XXH_X86DISPATCH_ALLOW_AVX)
 #  error "Error: if xxh_x86dispatch.c is compiled with AVX enabled, the resulting binary will crash on sse2-only cpus !! " \
          "If you nonetheless want to do that, please enable the XXH_X86DISPATCH_ALLOW_AVX build variable"
 #endif
+#endif
 
 #ifdef __has_include
 #  define XXH_HAS_INCLUDE(header) __has_include(header)