OFConstantString: _cStringLength -> unsigned int.

This field is actually only an unsigned int and not a size_t.

It never caused trouble on 64-bit little endian machines as padding was
added and so the most significant 32 bits were just 0. However, on 64-bit
big endian machines like SPARC64, the most significant 32 bit come
first, and thus the 32 bit size would treated as the 32 bit most
significant bits of a 64 bit value.

This also adds OpenBSD 5.3/SPARC64 to PLATFORMS.md.