[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.
> 
> I hope this remains on schedule. I want to remove AlignedBundling support from MCAssembler; its presence has made improving fragment relaxation within the assembler exceptionally difficult.

Concrete benefits:

* If we can remove `uint8_t BundlePadding` from `MCDataFragment`, `sizeof(MCDataFragment)` can decrease from 64 (https://github.com/llvm/llvm-project/pull/146307) to 56, yielding more max-rss saving.
* We can likely delete `MCELFStreamer::emitInstToData` and use the base [`MCObjectStreamer::emitInstToData`](cd075a40133dbe8a01f4fb1d6b99d55869089edf). 

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