[cesarb]

For AESNI, you probably are already using some sort of assembly to call the instructions. In the same assembly, you could wipe the key and plaintext as the last step.

For the stack, if you can guess how large the function's stack allocation can be (shouldn't be too hard for most functions), you could after returning from it call a separate assembly function which allocates a larger stack frame and wipes it (don't forget about the redzone too!). IIRC, openssl tries to do that, using an horrible-looking piece of voodoo code.

For the registers, the same stack-wiping function could also zero all the ones the ABI says a called function can overwrite. The others, if used at all by the cryptographic function, have already been restored before returning to the caller.

Yes, it's not completely portable due to the tiny amount of assembly; but the usefulness of portable code comes not from it being 100% portable, but from reducing the amount of machine- and compiler-specific code to a minimum. Write one stack- and register-wipe function in assembly, one "memset and I mean it" function using either inline assembly or a separate assembly file, and the rest of your code doesn't have to change at all when porting to a new system.