[OpenMP][JIT] Cleanup JIT interface, caching, and races

The JIT interface was somewhat irregular as it used multiple global
functions. It also did not cache the results of the JIT, hence multiple
GPU systems would perform the work multiple times. Finally, there might
have been races on the state if we have multi-threaded initialization of
different embedded images, or one image initialized on multiple devices.

This patch tries to rectify all of the above. The JITEngine is now a
part of the GenericPluginTy and tied to one target triple. To support
multiple "ComputeUnitKind"s (previously confusingly called Arch or
[M]CPU) and to avoid re-jitting for the same ComputeUnitKind, we keep a
map of JIT results per ComputeUnitKind. All interaction with the JIT
happens through the JITEngine directly, two functions are exposed. Both
use (shared) locks to avoid races and cache the result. All JIT-related
environment variables are now defined together.

Differential Revision: https://reviews.llvm.org/D141081

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