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 

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.

When compiling on ZLinux, we got this error:

/llvm-project/llvm/examples/OrcV2Examples/LLJITWithRemoteDebugging/ \
RemoteJITUtils.h:80:65:   required from here...
/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of
'sizeof' to incomplete type 'llvm::orc::RemoteExecutorProcessControl'
  static_assert(sizeof(_Tp)>0,

This patch just removes nullptr from the initialization of
std::unique_ptr<RemoteExecutorProcessControl> to avoid the issue.

Patch by Tung D. Le (tung@jp.ibm.com). Thanks Tung!

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D107247

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

This is a first step towards consistently using the term 'executor' for the
process that executes JIT'd code. I've opted for 'executor' as the preferred
term over 'target' as target is already heavily overloaded ("the target
machine for the executor" is much clearer than "the target machine for the
target").