Fix linking errors with floorl on MSVC by including math.h in a .cpp file. floorl is a force inline function defined in a header thus not visible for the linker.

src/stage1/os.cpp

@@ -41,6 +41,7 @@
 #include <io.h>
 #include <fcntl.h>
 #include <ntsecapi.h>
+#include <math.h>
 
 // Workaround an upstream LLVM issue.
 // See https://github.com/ziglang/zig/issues/7614#issuecomment-752939981