Fix register argument passing in cross-compiled code when host == x86

The presence of the ARGUMENTS_IN_REGISTERS pre-processor macro suggests that cross-compiled code targeting ARMv7, x86-64, ARM64 or any other architecture with register argument passing will not work, as the macro is defined depending on host parameters.

The macro is used to retrieve the engine register, so this should be fixed by making the retrieval depend on TargetPlatform parameters that are subject to the template specialization.

This is a blocker because the code will likely crash otherwise.