Debian has support for 3 different arm machine and probably one which is
failing is a pretty old version which doesn't support the said instruction.

So we can make it specific to _aarch64_ (as per the arm official reference
manual ISB should be defined by all ARMv8 processors). [as per the ARMv7 specs
even 32 bits processor should support it but not sure which exact version
Debian has under armel].

In either case, the said performance issue will have an impact mainly with a
high-end processors with extreme parallelism so it safe to limit it to _aarch64_.

Corrects: MDEV-24630