[lld] [LLD] Ensure explicit PT_GNU_RELRO has correct p_align (PR #133022)

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bd1976bris wrote:

> Thanks for sharing this intriguing case! However, I’m not convinced this is a regression related to PHDRS specifying PT_GNU_RELRO. In both musl and glibc, the p_align value of PT_GNU_RELRO is simply ignored.
> 
> For situations like this, where the correct behavior isn’t entirely clear, I lean toward simplifying the code. The alignment of PT_GNU_RELRO, when defined in PHDRS, should match that of other program headers. There doesn’t seem to be a compelling reason to handle it differently.

Thanks @MaskRay - however, I see the situation differently:
 
With `gnu-ld`, `p_align` for `PT_GNU_RELRO` has always been set to `1`, dating back to the original introduction of this feature in 2004. That's also the case when the `PT_GNU_RELRO` is specified explicitly in a `PHDRS` command  for `gnu-ld`. So, `p_align = 1` can reasonably be considered the de facto standard. Moreover, it seems *wrong* that explicitly specifying `PT_GNU_RELRO` through a `PHDRS` command would result in behavior different from when the linker implicitly creates it.

Linkers don't offer a mechanism to set the `p_align` value, so forcing it to 1 requires a linker patch.

In the interest of transparency, I would like to note that I care about this issue because we have an internal spec where PT_GNU_RELRO.p_align is 1, based on LLD's behaviour at the time. I would like to apologize that this issue wasn't raised at the time the change was made.
 
Perhaps one solution would be to unify the explicit and implicit code paths for `PT_GNU_RELRO`? A more consistent way to handle `PT_GNU_RELRO` with a `PHDRS` command (IMO) would be to allow users to specify it with the following restrictions:
1. Only one `PT_GNU_RELRO` `phdr` is allowed.
2. Explicit assignment to this `phdr` is disallowed; instead, just as in the implicitly created case, any sections between `DATA_SEGMENT_ALIGN` and `DATA_SEGMENT_RELRO_END` would be implicitly assigned to it.

If we want to do the above, then fixing the `p_align` value for `PT_GNU_RELRO` now would ensure no behavior change when we implement the unification.

https://github.com/llvm/llvm-project/pull/133022