Use OPENSSL_64_BIT in rc4.c.

This avoids unnecessary differences between LP64 and LLP64. Also
MSVC throws overflow warnings in the big-endian 64-bit codepath,
so use the preprocessor.

Change-Id: I74cef2d631d39f282177e043ed24bc6ecbbcb8fd
Reviewed-on: https://boringssl-review.googlesource.com/1860
Reviewed-by: Adam Langley <agl@google.com>