[clang] [llvm] Remove Native Client support (PR #133661)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Thanks for working on this! This will be the first time I'm not going to oppose an effort to remove Native Client support 🎉 Although I am going to ask you to wait a couple of months to land it, until we finally turn it off for good and start deleting the support code from Chromium. This is planned after the M139 branch in late June. I will also give this a good review.

Thanks!

After this change goes through, we should look to eliminate the BundleAlignSize/`.bundle_align_mode` logic. This assembler code has introduced considerable difficulties when attempting to improve the MCAssembler, especially in functions like `ensureValid` and `MCELFStreamer::emitInstToData` where the layout calculation and bundling logic have shown to be particularly complex. 

https://github.com/llvm/llvm-project/pull/133661