[Rocprofiler-system]: Fix GPU event enumeration for rocprof-sys-avail and CLI option for parsing GPU HW Counters (#2476)

## Motivation

The `rocprof-sys-avail -H -c GPU` command is returning blank output which is expected to display a list of available GPU hardware counters instead.
The `rocprof-sys-sample` and `rocprof-sys-run` is missing the `--gpu-events` option for specifying GPU counter events during profiling.

## Technical Details

The initialize_event_info() function had a logic bug where it only called set_agents() if the agent_manager was empty, but the actual issue was that the gpu_agents and cpu_agents vectors were empty even when agents were discovered.
Fixed the conditional logic to properly call set_agents() when gpu_agents and cpu_agents are empty, regardless of the agent_manager state.

Added the `--gpu-events (-G)` option which sets the `ROCPROFSYS_ROCM_EVENTS` environment variable to the specified values.

Fixes an issue where unsupported GPU/APU arch is being skipped gracefully - more details about this issue in the below comment.

projects/rocprofiler-systems/source/bin/rocprof-sys-sample/impl.cpp

@@ -837,6 +837,18 @@ parse_args(int argc, char** argv, std::vector<char*>& _env)
                                            original_envs);
         });
 
+    parser
+        .add_argument({ "-G", "--gpu-events" },
+                      "Set the GPU hardware counter events to record (ref: "
+                      "`rocprof-sys-avail -H -c GPU`)")
+        .action([&](parser_t& p) {
+            auto _events =
+                join(array_config{ "," }, p.get<std::vector<std::string>>("gpu-events"));
+            rocprofsys::common::update_env(_env, "ROCPROFSYS_ROCM_EVENTS", _events,
+                                           update_mode::REPLACE, ":", updated_envs,
+                                           original_envs);
+        });
+
     parser.start_group("MISCELLANEOUS OPTIONS", "");
     parser
         .add_argument({ "-i", "--inlines" },