PlatformDarwinKernel calls the ctor directly, not setting no-jit

Fix a small thinko in https://reviews.llvm.org/D133534 .  Normally
DynamicLoaderDarwinKernels are created via the CreateInstance plugin
method, and that plugin method sets the Process CanJIT to false.
In the above patch, I added a new code path that can call the
DynamicLoaderDarwinKernel ctor directly, without going through
CreateInstance, and CanJIT was not being correctly set for the
process.

rdar://101148552