Add eld as default linker for Qualcomm Hexagon compiler

[quic-k]

Qualcomm opensourced their linker recently https://github.com/qualcomm/eld
This PR sets eld as the default linker for Qualcomm's Hexagon compiler.

[quic-k]

hi @dcbaker
2 checks fail, but I don't think that is due to my changes

[dcbaker]

We can't change the linker ID, that's a part of the DSL, with compiler.get_linker_id().

It also doesn't look like eld is just a rename of their existing qlcd linker, but it's a completely different linker.

So, we should have a new representation for that, instead of renaming the old one. Also, please, add the new representation in one patch, and use it in a second patch. There's some documentation changes for this too.

[quic-k]

thanks for the quick review
yes, a new representation for eld makes sense
I have added eld as a new linker in the first commit and used it for Qualcomm's Hexagon compiler in the second commit
also, let know what docs changes, if any, are required

[dcbaker]

This all looks good, thanks.

There's a page called the reference table (can't remember what the .md name is) that has a table of linker id's, that needs to be updated. Also, a release snippet that support was added would be good.

[quic-k]

thanks
addressed documentation changes

[dcbaker]

Thanks for making all of the changes!

@thesamesam, any more comments, or are you happy to merge?

[thesamesam]

Looks good to me now! Please do merge.

[eli-schwartz]

Sone of these changes look like they're all the same logical unit of change, and should be squashed into a single logical commit. e.g. documentation changes should be one change.

(It is not clear to me whether the first two commits are doing one thing -- adding support for detecting eld -- or two things -- adding support for eld and making meson switch the default linker to use.)