[llvm] [offload][omp] Move reading _kernel_environment to libomptarget (PR #222606)

[Alex Duran via llvm-commits]

adurang wrote:

After merging#223772, I have removed the host special handling that was needed before.

I think that was the major standing issue for this PR. @jhuber6 @ro-i

https://github.com/llvm/llvm-project/pull/222606