[lld] [ELF] Implement --force-group-allocation (PR #94704)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> First, thank you so much for doing this. :)

Np:)
 
> I applied this patch to 17.0.6 to compare behavior. The group sections portion appears to work as expected, but I'm confused by one aspect of the relocation section combining:

You need commit b8dface221f4490933b0d39deb769e97ca134e5f and 0930f62cf600d9e2e9a45fef1b3a422d50be89d5 to make the `.rela.*` section name match its relocated section.

Static relocation sections copied to the output due to -r/--emit-relocs cannot be matched by input section descriptions.
`.rela.text : { *(.rela.text .rela.text.* .rela.gnu.linkonce.t.*) }` is ignored. I think GNU ld ignores them as well.

https://github.com/llvm/llvm-project/pull/94704