[0/4] Add SVE support for load/store_lanes

[Richard Sandiford]

This mixture of target-specific and target-independent patches adds
support for SVE LD[234] and ST[234].  The main difference from
Advanced SIMD is that SVE uses an extended vector mode for the
array of vectors, instead of the integer modes used by Advanced SIMD.

Tested on aarch64-linux-gnu (without and without SVE), x86_64-linux-gnu
and powerpc64-linux-gnu.  OK to install?

Thanks,
Richard