[FMV] Unify ssbs and ssbs2.

According to https://developer.arm.com/documentation/102105/latest Arm Architecture Reference Manual for A-profile architecture: Known issues

2.206 D22789 In section C5.2.25 "SSBS, Speculative Store Bypass Safe", under the heading 'Configurations', the text that reads:

"This register is present only when FEAT_SSBS is implemented. Otherwise, direct accesses to SSBS are UNDEFINED."

is changed to read:

"This register is present only when FEAT_SSBS2 is implemented. Otherwise, direct accesses to SSBS are UNDEFINED."

This suggests that it's not worth splitting FEAT_SSBS2 from FEAT_SSBS in the compiler, since FEAT_SSBS cannot be used for predicating the MRS/MSR instructions. Those can access PSTATE.SSBS only when FEAT_SSBS2 is available. Moreover, there are no hardware implementations which implement FEAT_SSBS without FEAT_SSBS2, therefore unifying these features in the specification should not be a regression for feature detection.

name: Pull request about: Technical issues, document format problems, bugs in scripts or feature proposal.

Thank you for submitting a pull request!

If this PR is about a bugfix:

Please use the bugfix label and make sure to go through the checklist below.

If this PR is about a proposal:

We are looking forward to evaluate your proposal, and if possible to make it part of the Arm C Language Extension (ACLE) specifications.

We would like to encourage you reading through the contribution guidelines, in particular the section on submitting a proposal.

Please use the proposal label.

As for any pull request, please make sure to go through the below checklist.

Checklist: (mark with X those which apply)

[ ] If an issue reporting the bug exists, I have mentioned it in the PR (do not bother creating the issue if all you want to do is fixing the bug yourself).
[ ] I have added/updated the SPDX-FileCopyrightText lines on top of any file I have edited. Format is SPDX-FileCopyrightText: Copyright {year} {entity or name} <{contact informations}> (Please update existing copyright lines if applicable. You can specify year ranges with hyphen , as in 2017-2019, and use commas to separate gaps, as in 2018-2020, 2022).
[ ] I have updated the Copyright section of the sources of the specification I have edited (this will show up in the text rendered in the PDF and other output format supported). The format is the same described in the previous item.
[ ] I have run the CI scripts (if applicable, as they might be tricky to set up on non-*nix machines). The sequence can be found in the contribution guidelines. Don't worry if you cannot run these scripts on your machine, your patch will be automatically checked in the Actions of the pull request.
[ ] I have added an item that describes the changes I have introduced in this PR in the section Changes for next release of the section Change Control/Document history of the document. Create Changes for next release if it does not exist. Notice that changes that are not modifying the content and rendering of the specifications (both HTML and PDF) do not need to be listed.
[ ] When modifying content and/or its rendering, I have checked the correctness of the result in the PDF output (please refer to the instructions on how to build the PDFs locally).
[ ] The variable draftversion is set to true in the YAML header of the sources of the specifications I have modified.
[ ] Please DO NOT add my GitHub profile to the list of contributors in the README page of the project.

ARM-software / acle

[FMV] Unify ssbs and ssbs2. #350