Add SVE nontemporal stores and scatters, including masked variants

@holzer pointed out that SVE has nontemporal stores, which I overlooked when I implemented SVE support three years ago. So there are actually eight different kinds of stores we have to support (store, stream, mask-store, mask-stream, scatter, stream-scatter, mask-scatter, mask-stream-scatter).

(Mask-)stream-scatter requires SVE2; adding support for that introduced changes to a number of unrelated files. SVE2 is a superset of SVE, so the automatic detection won‘t return SVE if SVE2 is also available.

The added test coverage revealed a few bugs on master that I also fixed:

• We used an incorrect argument order for maskStoreS on RISC-V-V. This meant that maskStoreS just wouldn't compile on RISC-V-V.
• Our emulation of maskStore (via blendv) on ARM Neon and POWER VSX was zeroing the masked-out elements when nontemporal mode was selected (i.e. which due to the lack of real nontemporal stores maps to an emulation via cacheline zeroing). This lead to incorrect results.
• Our emulation of maskStore on POWER VSX was ignoring the mask for the last vector of each cacheline. This lead to even more incorrect results when nontemporal mode was selected.