[libomptarget][amdgpu] Call into deviceRTL instead of ockl

[libomptarget][amdgpu] Call into deviceRTL instead of ockl

Amdgpu codegen presently emits a call into ockl. The same functionality
is already present in the deviceRTL. Adds an amdgpu specific entry point
to avoid the dependency. This lets simple openmp code (specifically, that
which doesn't use libm) run without rocm device libraries installed.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D93356