would -DARM_ENABLED=${BLENDER_PLATFORM_ARM} work for all platforms? would allow some deduplication of two identical looking blocks.

otherwise something like this would be my preferred way to go about this (not tested, i may have screwed up the syntax, test before blindly lifting this code)

elseif(UNIX)
    set(ISPC_EXTRA_ARGS_UNIX
      -DCMAKE_C_COMPILER=${LIBDIR}/llvm/bin/clang
      -DCMAKE_CXX_COMPILER=${LIBDIR}/llvm/bin/clang++
      -DFLEX_EXECUTABLE=${LIBDIR}/flex/bin/flex
    )
  if(BLENDER_PLATFORM_ARM)
    LIST(APPEND ISPC_EXTRA_ARGS_UNIX -DARM_ENABLED=On)
  endif()

I was concerned that wouldn't work because "BLENDER_PLATFORM_ARM" is not defined at all by default, it is only defined (and set to On) when on an ARM system, so CMake would probably misinterpret this on non-ARM systems.
And ISPC CMakeLists.txt defaults ARM_ENABLED to On, so would actually have to explicitly disable it on x86, rather than enable it on ARM. But since that might be changed, I thought it safer to simply explicitly set it in both cases.