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

test/libc/calls/mkntpath_test.c

@@ -18,8 +18,10 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/dce.h"
 #include "libc/mem/gc.internal.h"
 #include "libc/testlib/testlib.h"
+#if SupportsWindows()
 
 char16_t p[PATH_MAX];
 
@@ -48,3 +50,5 @@ TEST(mkntpath, testRemoveDoubleSlash) {
   EXPECT_EQ(21, __mkntpath("C:\\Users\\jart\\\\.config", p));
   EXPECT_STREQ(u"C:\\Users\\jart\\.config", p);
 }
+
+#endif /* SupportsWindows() */