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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Test added. Does that look right?

Yes, that looks quite good, thanks!

> I kind of expected `.data_cygwin_nocopy` between `__data_end__` and `__bss_start__` but I don't think it matters.

Hmm, right - doesn't this mean that we end up including the BSS zero bytes in the actual linked file, while the normal idea is to order them at the end of `.data` so they can be omitted from the actual bytes on disk. So it may be worthwhile trying to reorder this. I wonder if switching the order of the `parseMerge` calls is enough?

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