message: added ENOKI_REGCALL define for 'regcall' calling conventions
diff: # define ENOKI_PACK
  # define ENOKI_LIKELY(x) x
  # define ENOKI_UNLIKELY(x) x
  +# define ENOKI_REGCALL
  #else
  # define ENOKI_NOINLINE __attribute__ ((noinline))
  # define ENOKI_INLINE __attribute__ ((always_inline)) inline
  # define ENOKI_NOUNROLL _Pragma("nounroll")
  # define ENOKI_IVDEP
  # define ENOKI_MAY_ALIAS __attribute__ ((may_alias))
  +# define ENOKI_REGCALL __attribute__ ((regcall))
  # elif defined(__INTEL_COMPILER)
  # define ENOKI_MAY_ALIAS
  # define ENOKI_UNROLL _Pragma("unroll")
  # define ENOKI_NOUNROLL _Pragma("nounroll")
  # define ENOKI_IVDEP _Pragma("ivdep")
  +# define ENOKI_REGCALL __attribute__ ((regcall))
  # else
  # define ENOKI_MAY_ALIAS
  # define ENOKI_UNROLL
  # else
  # define ENOKI_IVDEP
  # endif
  +# define ENOKI_REGCALL
  # endif
  #endif
  