The open source XNU does not support special handling of JIT memory.  However, macOS Safari and WebKit binaries still need to be able to run on the open source XNU, albeit without JIT.

However, ExecutableAllocator::disableJIT() is still trying to allocated JIT memory if the JIT entitlement is present (which is defined by the WebKit binary, not the kernel), and currently, will assert that the allocation succeeds.  We need to skip this allocation and assertion when running on open source XNU because the allocation is not necessary, and the assertion is invalid for the open source kernel and will fail every time.