[Bug target/64782] -mcpu=native should be supported on aarch64

["ktkachov at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64782

--- Comment #4 from ktkachov at gcc dot gnu.org ---
(In reply to James Greenhalgh from comment #3)
> Did the Linux kernel format for /proc/cpuinfo settle down enough that we can
> sensibly parse it [1]?
> 
> When implementing this support I guess we will want to look at which kernels
> are going to be out there in use, and possibly even try to detect the
> /proc/cpuinfo format dynamically (Ugh!). Perhaps you just want to mandate a
> minimum kernel version, and bail out if you see something older?
> 
> [1]: https://lkml.org/lkml/2014/10/24/391

That's the format in the link that I was thinking of supporting.
The format before the one proposed in the link would work as well, but only for
detecting the native -march or non-big.LITTLE systems.

But since we would want consistent behaviour regardless of kernel version, we
should probably mandate a minimum kernel version for this feature.