[Libomptarget] Correctly implement `getWTime` on AMDGPU

AMDGPU provides a fixed frequency clock since some generations back.
However, the frequency is variable by card and must be looked up at
runtime. This patch adds a new device environment line for the clock
frequency so that we can use it in the same way as NVPTX. This is the
correct implementation and the version in ASO should be replaced.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D154456