os::stat() on Posix platform does not need to copy input path

1) os::stat() on all Posix platforms copies the input path into a fixed sized temp buffer, thereby risking truncation, only to call os::native_path() which is a noop on all Posix platforms.

Just call ::stat() directly, no need for the buffer.

Also, the coding can be unified in os_posix.cpp.

2) os::native_path() exists for the sole purpose of making windows happy, this should be a windows only function (os::win32) and be removed from os.hpp. A comment would be helpful.