ci: Test aarch64 with QEMU, with varying SVE vector lengths
Also add build testing with a recent aarch64 Clang.
CC @arpadpanyik-arm - this testing of multiple SVE vector lengths is pretty much essential for being able to accept any SVE code, otherwise we could have latent bugs that would only surface on newer HW with longer vectors.
CC @another - who touched lots of similar CI lately.