[clang] [Hexagon] Pass -m emulation flag to linker from clang driver (PR #185745)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > ```
> > $ ld.eld hw.o && llvm-readelf -l -W a.out 
> >   LOAD           **0x001000** 0x00000000 0x00000000 ....
> > 
> > $ld.eld -mhexagonlinux hw.o && llvm-readelf -l -W a.out
> >   LOAD           **0x000000** 0x00000000 0x00000000 ....
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > Can we have a switch such as `-z programheader` that linker can consume to selectively load program headers when linker script is not used ?
> 
> I think @quic-seaswara was asking @MaskRay here if this was an appropriate solution for consideration by the linkers.
> 
> If that works, then I'll update this PR to use that new switch instead.

I am still confused. Can you paste `hw.o` and show the full llvm-readelf -Sl output?

lld's program header allocation logic is at LinkerScript::allocateHeaders in case that helps.

https://github.com/llvm/llvm-project/pull/185745