[libomptarget][nfc] Refactor amdgpu partial barrier to simplify adding a second one

[libomptarget][nfc] Refactor amdgpu partial barrier to simplify adding a second one

D101976 would require a second barrier instance. This NFC to amdgpu makes it
simpler to add one (an extra global, one more line in init). Also renames the
current barrier to L0.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D102016