Should lld support binary output ("--oformat binary")?

Maybe we should implement it. I had a concern about adding complexity for
the less frequently used feature, but I believe it can be implemented
easily. Specifically, I think we need to

> - implement another version of `Writer<ELFT>::writeSections` for

--oformat=binary and

> - stop calling `Writer::writeHeaders` and `writeBuildId` if

--oformat=binary is specified.

Patch implementing --oformat binary is here: https://reviews.llvm.org/D23769
I noticed that ld and gold has idfferent behavior:
gold uses LMA of PT_LOADs to find the destination file offset of data which it takes from segments.
I tried to play with AT() and looks GNU ld ignores that.

Patch implements ld behavior for now.

George.