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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Not too horribly ugly?

Hmm, that's not too bad indeed.

Before going ahead and blessing that approach, I'd like a second opinion from someone else though - is the second commit here acceptable to @cjacek, @aganea, @rnk or others? 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?

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