RFR: 8352098: -Xrunjdwp fails on static JDK

[Jiangli Zhou]

Please review this fix that avoids `JvmtiAgent::convert_xrun_agent` from prematurely exiting VM if `lookup_On_Load_entry_point` cannot load the agent using `JVM_OnLoad` symbol. Thanks

`lookup_On_Load_entry_point` first tries to load the builtin agent from the executable by checking the requested symbol (`JVM_OnLoad`). If no builtin agent is found, it then tries to load the agent shared library (e.g. `libjdwp.so`) by calling `load_library`. The issue is that `load_library` is called with `vm_exit_on_error` set to `true`, which causes the VM to exit immediately if the agent shared library is not loaded. Therefore, `JvmtiAgent::convert_xrun_agent` has no chance to try loading the agent using `Agent_OnLoad` symbol (https://github.com/openjdk/jdk/blob/19154f7af34bf6f13d61d7a9f05d6277964845d8/src/hotspot/share/prims/jvmtiAgent.cpp#L352). This is a hidden issue on regular JDK, since the `load_library` can successfully find the agent shared library when `JvmtiAgent::convert_xrun_agent` first tries to load the agent using `JVM_OnLoad` symbol. The issue is noticed on static JDK as there is no `libjdwp.so` in static JDK. It can be reproduced with jtreg `runtime/6294277/Sourc
 eDebugExtension.java` test.

As part of the fix, I cleaned up following in `invoke_JVM_OnLoad` and `invoke_Agent_OnLoad`. If there's an error, the VM should already have exited during `lookup_<JVM|Agent>_OnLoad_entry_point` in those cases.


  if (on_load_entry == nullptr) {
    vm_exit_during_initialization("Could not find ... function in -Xrun library", agent->name());
  }

-------------

Commit messages:
 - Add 'assert(on_load_entry != nullptr, "invariant");'
 - Cleanup
 - Return nullptr if lookup_On_Load_entry_point does not load agent from executable or library.
 - When called from JvmtiAgent::convert_xrun_agent, don't report any error and bail out too early in lookup_JVM_OnLoad_entry_point if it does not succeed, since we want to try lookup_Agent_OnLoad_entry_point for Agent_OnLoad as well.

Changes: https://git.openjdk.org/jdk/pull/24086/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24086&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352098
  Stats: 34 lines in 1 file changed: 12 ins; 5 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/24086.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24086/head:pull/24086

PR: https://git.openjdk.org/jdk/pull/24086