[libomptarget] Move device environment to shared header, remove divergence

Follow on to D110006, related to D110957

Where implementations have diverged this resolves to match the new DeviceRTL

- replaces definitions of this struct in deviceRTL and plugins with include
- changes the dynamic_shared_size field from D110006 to 32 bits
- handles stdint being unavailable in DeviceRTL
- adds a zero initializer for the field to amdgpu
- moves the extern declaration for deviceRTL to target_interface
  (omptarget.h is more natural, but doesn't work due to include order
  with debug.h)
- Renames the fields everywhere to match the LLVM format used in DeviceRTL
- Makes debug_level uint32_t everywhere (previously sometimes int32_t)

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D111069