gdb/testsuite/rocm.exp: Use system GPU(s) to detect features

gdb/testsuite/rocm.exp: Use system GPU(s) to detect features

gdb/testsuite/rocm.exp: Use system GPU(s) to detect features

Background
----------
This patch revisits the purpose of hcc_amdgpu_targets{} in
order to address the separation of concerns between:

- GPU targets passed to the compiler.  This kind of target
  is passed as an argument to flags like "--offload-arch=...",
  "--targets=...", etc.

- GPU targets as in available GPU devices on the system.  This
  is crucial for finding which capabilities are available,
  and therefore which tests should be executed or skipped.

Code change
-----------
- A new "find_amdgpu_devices{}" procedure is added.  It is
  responsible for listing the GPU devices that are available
  on the system.

- "hcc_amdgpu_targets{}" is rewritten to use the newly added
  "find_amdgpu_devices{}" when there's no environment variable
  (HCC_AMDGPU_TARGET) set.

- The output of "hcc_amdgpu_targets{}" is now only used in
  places that set the target for the building toolchains.

- The output of "find_amdgpu_devices{}" is used anywhere that
  needs to evaluate the GPU features.

Approved-By: Lancelot Six <lancelot.six@amd.com> (amdgpu)
Change-Id: Ib11021dbe674aa40192737ede78284a1bc531513