Make std.file.timeLastModified safe

On Windows systems, it does not contain any unsafe operations.
On Posix systems, it contains the following unsafe operations but we can verify they can be trusted.

- use of `core.sys.posix.sys.stat.stat`
- use of `std.internal.cstring.tempCString` and `tempCString.Res.~this`
- taking an address of a local variable (`statbuf`)