[llvm] [MC] Add .loc_label instruction (PR #99710)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> You could emit the label difference into some other attribute in assembly rather than in an instruction (guess that's not necessarily better - it'll necessarily be a bogus attribute/value combination I guess) - or I guess just into its own section and dump the bytes of the section raw...
> 
> Guess you could use an unknown/bogus attribute number, I think it'd still dump OK & we'd just print a bogus attribute number in the attribute name somehow... - mixed feelings about what the best way to do that is.

It's usually better to inspect the symbol table with `llvm-readelf -s` to know other properties (e.g. binding/type, even if they are the default, STB_LOCAL, STT_NOTYPE). A label difference could lead to both relocations and a content change. 

https://github.com/llvm/llvm-project/pull/99710