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

[Peter Smith 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;
+
----------------
smithp35 wrote:

Alternatively it should be possible to universally set SHF_AARCH64_PURECODE and then
handle this in `Writer.cpp::createPhdrs()`

https://github.com/llvm/llvm-project/blob/main/lld/ELF/Writer.cpp#L2381
```
    uint64_t newFlags = computeFlags(ctx, sec->getPhdrFlags());
    // When --no-rosegment is specified, RO and RX sections are compatible.
    uint32_t incompatible = flags ^ newFlags;
    if (ctx.arg.singleRoRx && !(newFlags & PF_W))
      incompatible &= ~PF_X;
```

Something like:
```
  if (sec == ctx.in.plt && (flags & PF_R))
    newFlags |= PF_R;
```

It is true that the .plt could in theory be the first section, but this would normally take a linker script making it the first OutputSection, but I think that's unlikely, and could be fixed with PHDRS.

I did think we might do this for all OutputSections but I guess for bare-metal there's still a use case for separate XO and non-XO segments.

Another possibility is to record any non-XO OutputSection that we see in ctx.

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