Use constexpr for register constants

[danluu]

inc/NativeJIT/Nodes/ParameterNode.h

        // Integer parameters are passed in RCX, RDX, R8, and R9.
        // Use constants to encode registers. See #31.
#ifdef NATIVEJIT_PLATFORM_WINDOWS
        const uint8_t idMap[] = {1, 2, 8, 9};
#else
        const uint8_t idMap[] = {7, 6, 2, 1, 8, 9};
#endif

See also #24