There is no need to ifdef anything, the memcpy is the only correct way to do it. As memcpy is also a reserved identifier in C, the compiler can and will optimize this into a 64-bit access on those platforms where it can be safely done so (x86 for example), e.g. GCC compiles

    uint64_t func(char *buf) {
        uint64_t rv;
        memcpy(&rv, buf+3, sizeof(rv));
        return rv;
    }

into

    movq    3(%rdi), %rax
    ret

On Linux 64-bit ABI.