power10: strncmp fails to save and restore nonvolatile vector registers

The Power 10 implementation of strncmp in
sysdeps/powerpc/powerpc64/le/power10/strncmp.S failed to save/restore
nonvolatile vector registers in the 32-byte aligned loop path.  This
results in callers reading content from those registers in a different
context, potentially altering program logic.

There could be a program context where a user controlled string could
leak through strncmp into program code, thus altering its logic.  There
is also a potential for sensitive strings passed into strncmp leaking
through the clobbered registers into parts of the calling program that
should otherwise not have had access to those strings.

The impact of this flaw is limited to applications running on Power 10
hardware that use the nonvolatile vector registers, i.e. v20 to v31
assuming that they have been treated in accordance with the OpenPower
psABI.  It is possible to work around the issue for those specific
applications by setting the glibc.cpu.hwcaps tunable to "-arch_3_1" like
so:

    export GLIBC_TUNABLES=glibc.cpu.hwcaps=-arch_3_1

CVE-id: CVE-2025-5745
Public-Date: 2025-06-05
Vulnerable-Commit: 23f0d81608d0ca6379894ef81670cf30af7fd081 (2.40)
Fix-Commit: 63c60101ce7c5eac42be90f698ba02099b41b965 (2.42)
Fix-Commit: 84bdbf8a6f2fdafd3661489dbb7f79835a52da82 (2.41-57)
Fix-Commit: 42a5a940c974d02540c8da26d6374c744d148cb9 (2.40-136)