[libomptarget][nfc] Wrap cuda min() in target_impl

Summary:
[libomptarget][nfc] Wrap cuda min() in target_impl

nvptx forwards to cuda min, amdgcn implements directly.
Sufficient to build parallel.cu for amdgcn, added to CMakeLists.

All call sites are homogenous except one that passes a uint32_t and an
int32_t. This could be smoothed over by taking two type parameters
and some care over the return type, but overall I think the inline
<uint32_t> calling attention to what was an implicit sign conversion
is cleaner.

Reviewers: ABataev, jdoerfert

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71580