[FMV] Unify ssbs and ssbs2. (#350)

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.

main/acle.md

@@ -413,6 +413,10 @@ Armv8.4-A [[ARMARMv84]](#ARMARMv84). Support is added for the Dot Product intrin
 * Added [**Alpha**](#current-status-and-anticipated-changes)
   support for modal 8-bit floating point intrinsics.
 
+#### Changes for next release
+
+* Unified Function Multi Versioning features ssbs and ssbs2.
+
 ### References
 
 This document refers to the following documents.
@@ -2815,8 +2819,7 @@ The following table lists the architectures feature mapping for AArch64
    | 460           | `FEAT_MTE3`              | memtag3       | ```ID_AA64PFR1_EL1.MTE >= 0b0011```       |
    | 470           | `FEAT_SB`                | sb            | ```ID_AA64ISAR1_EL1.SB >= 0b0001```       |
    | 480           | `FEAT_SPECRES`           | predres       | ```ID_AA64ISAR1_EL1.SPECRES >= 0b0001```  |
-   | 490           | `FEAT_SSBS`              | ssbs          | ```ID_AA64PFR1_EL1.SSBS >= 0b0001```      |
-   | 500           | `FEAT_SSBS2`             | ssbs2         | ```ID_AA64PFR1_EL1.SSBS >= 0b0010```      |
+   | 490           | `FEAT_SSBS`, `FEAT_SSBS2`| ssbs          | ```ID_AA64PFR1_EL1.SSBS >= 0b0010```      |
    | 510           | `FEAT_BTI`               | bti           | ```ID_AA64PFR1_EL1.BT >= 0b0001```        |
    | 520           | `FEAT_LS64`, `FEAT_LS64_V`, <br> `FEAT_LS64_ACCDATA` | ls64 | ```ID_AA64ISAR1_EL1.LS64 >= 0b0011``` |
    | 550           | `FEAT_WFxT`              | wfxt          | ```ID_AA64ISAR2_EL1.WFxT >= 0b0010```     |