Some maintenance on implementation and tests:
* Drop manual TargetMachineBuilder setup
* Drop addDebugSupport() in favor of Orc's enableDebuggerSupport()
* Check that debug support plugins append jit_code_entry
* Update and reduce sample input 

https://github.com/llvm/llvm-project/pull/74764 reported that the
`lljit-with-remote-debugging` test fails on AArch64 hosts, because the
input IR file states arch x86_64 explicitly. In order to drop the target
triple we have to remove a check in the example implementation.

Not sure it's fully portable now, but at least it's better than before.

Clients don't care about linkage, and ExecutorAddr is much more ergonomic.

Though this is a full port of the example, it is not yet fully functional due to a threading issue in the SimpleRemoteEPC implementation. The issue was discussed in D110530, but it needs a more thorough solution. For now we are dropping the dependency to the old `OrcRPC` here (it's been the last use-case in-tree). The test for the example is under review in ... and will be re-enabled once the threading issue is solved.

The API change originated from D104694. The LLJITWithRemoteDebugging example and the test for it were disabled while it was in the works.