[LLD/ELF] Document large section support

[rnk]

LLD's support for the large (and medium) code models has evolved significantly in the last year, and I think it deserves a standalone writeup, similar to the other features which are written up in lld/ELF/docs: __start/__stop, backref warnings, and linker scripts.

In particular, a public diagram of the section layout (.rodata, .text, .data, .bss -> add in the .l variants where they go) that is linkable and searchable would be really helpful, along with an explanation of the concept of relocation pressure.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Reid Kleckner (rnk)

Details

LLD's support for the large (and medium) code models has evolved significantly in the last year, and I think it deserves a standalone writeup, similar to the other features which are written up in [lld/ELF/docs](https://github.com/llvm/llvm-project/tree/main/lld/docs/ELF): __start/__stop, backref warnings, and linker scripts.

In particular, a public diagram of the section layout (.rodata, .text, .data, .bss -> add in the .l variants where they go) that is linkable and searchable would be really helpful, along with an explanation of the concept of relocation pressure.