arm64/compat32: Fix handling of 32bits FP registers.

We must consider the aarch32 FP registers as 16 128bits registers, and store
that as the first 16 aarch64 FP registers.

PR: 267788

(cherry picked from commit ccd0f34d8585cba727dd17a381309855af655b82)
(cherry picked from commit 0e0a03c792542a2509702378559622efafc86548)
Approved by: re (cperciva)