arch: arm64: Implement SVE context switching for ARMv9-A

Implement Scalable Vector Extension (SVE) context switching support,
enabling threads to use SVE and SVE2 instructions with lazy context
preservation across task switches.

The implementation is incremental: if only FPU instructions are used
then only the NEON access is granted and preserved to minimize context
switching overhead. If SVE is used then the NEON context is upgraded to
SVE and then full SVE access is granted and preserved from that point
onwards.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>