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
This reapplies 85c649bc02a, which was reverted in 35767e43d62 due to failures
with some example programs. The fix was to add export_executable_symbols to the
example programs.
