[llvm] [X86Backend] Use GetExternalSymbolSymbol for MO_ExternalSymbol. (PR #133352)

[weiwei chen via llvm-commits]

weiweichen wrote:

This is another attempt to address the issue what was poorly addressed in https://github.com/llvm/llvm-project/pull/108880 🤦‍♀️ . Adding more explanation on the motivation/problem to address and the rationale on the change. Appreciate suggestions on how to add a test in addition to existing tests not being broken this change 🙏 

https://github.com/llvm/llvm-project/pull/133352