There is a lot more going on here then just compilers.
Linux features a robust 'Linux Kernel Cryptographic API' that is used by various programs to accelerate things. Features such as fast random number generations, cryptographic kernel modules, and that sort of thing.
One of it's original purposes was to accelerate IPSEC, which requires in-kernel cryptographic algorithm support to work properly. OpenSSL takes advantage of it when present.
This sort of thing, especially random number generation performance, is going to be my guess as to why Linux outclasses OS X by a wide margin.
It's theoretically possible that OpenCL might make a difference. But OpenSSL would have to be re-designed to take advantage of it.