[lld] [LLD][AArch64] Mark .plt with PURECODE flag if all input sections also have it (PR #132224)

[Csanád Hajdú via llvm-commits]

================
@@ -2610,6 +2610,18 @@ PltSection::PltSection(Ctx &ctx)
   // modify the instructions in the PLT entries.
   if (ctx.arg.emachine == EM_SPARCV9)
     this->flags |= SHF_WRITE;
+
----------------
Il-Capitano wrote:

Thanks for the thorough reply! It really helped refine my understanding of the problem.

You're right that the main use case we care about is the whole program being XO or RX. What do you think about doing the following:
* Unconditionally set `SHF_AARCH64_PURECODE` for `.plt`.
* When auto generating program headers, consider XO and RX sections compatible, allowing them to be placed in the same segment.
* At this point we don't need to strip the PURECODE flag from the output sections, they'll just be placed in a program header that is RX instead of XO. Leaving the section flag intact shouldn't cause any issues I think.

We can do this by just adding the following snippet in `createPhdrs()`:
```c++
    if (newFlags & PF_X)
      incompatible &= ~PF_R;
```

For bare-metal targets, this wouldn't allow separate auto-generated program headers with XO and RX code though, a linker script would be required to separate those out into different program headers. I don't have any experience working with bare-metal, do you think this is a reasonable requirement? If not, can we detect in the linker whether we're linking for a target with an OS or not?

If you think this would be a good approach, I'll open a separate PR superseding this one, as it's a more general solution.

https://github.com/llvm/llvm-project/pull/132224