[libc][NFC] remove dependency on non standard ssize_t

`ssize_t` is from POSIX and is not standard unfortunately.
Rewritting the code so it doesn't depend on it.

Differential Revision: https://reviews.llvm.org/D94760

GitOrigin-RevId: e517dff50a4f933d0729026901b14c7c1112a359
1 file changed