Platform: make MSVCRT more Unix-libc like
Since `stdin`, `stdout`, and `stderr` are defined via macros, we cannot see them
on the swift side. Replicate these by hand. Expose `STDIN_FILENO`,
`STDOUT_FILENO`, and `STDERR_FILENO` for compatibility with other libc
implementations. This reduces the need for changing the codebase unnecessarily
for MSVCRT.
