arm64 VEX frontend and backend support for Iop_M{Add,Sub}F{32,64}

arm64 VEX frontend and backend support for Iop_M{Add,Sub}F{32,64}

The arm64 frontend used to implement the scalar fmadd, fmsub, fnmadd
and fnmsub iinstructions into separate addition/substraction and
multiplication instructions, which caused rounding issues.

This patch turns them into Iop_M{Add,Sub}F{32,64} instructions
(with some arguments negated). And the backend now emits fmadd or fmsub
instructions.

Alexandra Hajkova <ahajkova@redhat.com> added tests and fixed up the
implementation to make sure rounding (and sign) are correct now.

https://bugs.kde.org/show_bug.cgi?id=426014