[lld] [LLD][COFF] add __{data,bss}_{start,end}__ symbols for Cygwin support (PR #136180)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> > As users already can specify the `-merge` option on the command line, perhaps that even would be worthwhile to split out to a separate PR, as it would be generally beneficial if a user wants to merge any other section into `.data` as well?
> 
> I wouldn't be opposed to that (and adding a separate test of that).

I think that'd be the clearest move here in any case; that should get enough visibility on its own, that I'd be comfortable approving it if it doesn't get feedback from others within a couple of days.

https://github.com/llvm/llvm-project/pull/136180