Fix building libstd on emscripten targets.
The main cause of the problem is that libstd/os/mod.rs treats emscripten targets as an alias of linux targets, whereas liblibc treats emscripten targets as musl-compliant, so it gets a slightly different struct stat64 defined.
This commit adds conditional compilation checks to use the correct timestamp format on fs metadata functions in the case of compiling to emscripten targets.
This commit also depends needs f1575cff2d applied in order to successfully build libstd with emscripten target.
4352a8554f