[libc][NFC] fix sprintf test on 32 bit systems

The expected number for the max ptrdiff value was expected to be exactly
4294967296 (2**32) for 32 bit systems, when it should be
4294967295 (2**32 - 1). This also adds a second test to check for this
case on non-32 bit systems.

Reviewed By: lntue, mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D156257