[lld] [LLD][ELF] Do not emit __start/__stop for empty sections (PR #96213)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > Behaviors of different visibilities are unified, even if the behavior differs from GNU ld.
> 
> Your suggested alternative causes the test `ELF/linkerscript/sections-gc2.s` to fail. How important this is, I'm not sure. Also the emission of the empty section in this test case doesn't feel quite right either.
> 
> If the concern is the performance impact of the addition of `if (sym->isLocal() && sym->isUndefined())` to the `.symtab` loop, then perhaps that can be dropped? However, doing so would leave behind the `LOCAL HIDDEN UND` `__start/__stop` symbols, but that's still better than symbols with invalid section indices.

Dropping `if (sym->isLocal() && sym->isUndefined())`  helps. With more thinking, I believe we should do #96343 instead. That patch simplifies the code,  fixes the incorrect section indexes, and unified the behavior with `__init_array_start`.

Aligning empty section behavior with GNU ld in this particular case isn't useful IMO.

https://github.com/llvm/llvm-project/pull/96213