[DebugInfo][AIX] Set target debugger-tune default to dbx

https://reviews.llvm.org/D99400 set clang DefaultDebuggerTuning for AIX
to dbx. However, we still need to update the target default so that llc
and other tools will get the same default debuggertuning, and avoid
passing extra options in LTO.

Reviewed By: #powerpc, shchenz, dblaikie

Differential Revision: https://reviews.llvm.org/D101197