1. Compile without optimization (maybe just the crypto parts, if that's possible).
2. Write all crypto stuff in assembly, link in as static binary.
W.r.t. 1, the compiler's definition of no optimization today is not the same thing as it was last version, or will be next version. For instance, on IA-64 there are things the compiler has to do that are typically considered optimizations.
W.r.t. 2, you have to make sure there is no link-time optimization happening.
For instance, when someone runs it in an emulator for backwards compatibility purposes. Or when someone runs it in a JITter. Or even just if the compiler decides to special-case for the existing link target.
However, that is not an inherent restriction - that is only a restriction on current compilers. It is entirely possible for a compiler to read the assembly of things being linked and optimize based on that.