EmitC: Add emitc.global and emitc.get_global (#145) (#88701)

This adds
- `emitc.global` and `emitc.get_global` ops to model global variables
similar to how `memref.global` and `memref.get_global` work.
- translation of those ops to C++
- lowering of `memref.global` and `memref.get_global` into those ops

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Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>