[llvm] [OpenMP][offload] Add LIBOMPTARGET_KERNEL_EXE_TIME (PR #222105)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

@shiltian not really sure what you mean by a plugin in this context, tbh.

I think that @mjklemm has a good point when he says that we could use a more comprehensive profiling env var.
So, maybe I/someone can do an extended version of this PR at some point.

At the moment, I'm fine with using downstream and/or patching the compiler for my current needs.

https://github.com/llvm/llvm-project/pull/222105