[amdgpu-arch] Dynamically load the HSA runtime if not found during the build

[amdgpu-arch] Dynamically load the HSA runtime if not found during the build

We use the `amdgpu-arch` tool to query the installed GPUs at runtime.
One problem is that this tool is currently not build if the person
building the LLVM binary does not have the HSA runtime on their system.
This means that if someone built and distrubted an installation of LLVM
without HSA, then the user will not be able to use it even if they have
it on their system.

This patch makes us build this tool unconditionally and adds extra logic
to dynamically load HSA if it's present.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D141859