[v0.30] Add uarch detection to AMD GPUs

Similarly to NVIDIA and Intel GPUs, we now detect microarchitecture,
also with manufacturing process and specific chip name. We infer all
of this from the gfx name (in the code we use the term llvm_target),
altough it's not clear yet that this method is completely reliable (see
comments for more details). In the future we might want to replace that
with a better way. Once we have the gfx name, we *should* be able to
infer the specific chip, and from the chip we can easily infer the
microarchitecture.

This commit also includes some refactorings and code improvements on
the HSA backend.

src/common/uarch.hpp

@@ -16,6 +16,9 @@ struct uarch {
   int32_t cc_minor;
   int32_t compute_capability;
 
+  // HSA specific
+  int32_t llvm_target;
+
   // Intel specific
   int32_t gt;
   int32_t eu;