aarch64/fpu: Sync libmvec routines from 2.39 and before with AOR

This includes a fix for big-endian in AdvSIMD log, some cosmetic
changes, and numerous small optimisations mainly around inlining and
using indexed variants of MLA intrinsics.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

(cherry picked from commit e302e102)