[Bug target/126935] [17 Regression] SVE2 incorrectly detected on aarch64 vm

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126935

--- Comment #3 from ktkachov at gcc dot gnu.org ---
(In reply to Drea Pinski from comment #2)
> Actually I think the better check is if the processor has SVE and SVE2.
> Even though SVE2 should be enough here; it seems like some VMs advertise
> SVE2 but not SVE support (which in itself odd but that ever).

Oof, SVE2 but not SVE is a nonsense configuration.
Anyway, for the libcpp fast-path I've added I think we can make the configure
check also execute the SVE2 instructions required, not just test their HWCAPs.
That would sidestep the dodgy VM advertising