Call `MulAddMul` instead of multiplication in _generic_matmatmul! (#56089)
Fix https://github.com/JuliaLang/julia/issues/56085 by calling a newly
created `MulAddMul` object that only wraps the `alpha` (with `beta` set
to `false`). This avoids the explicit multiplication if `alpha` is known
to be `isone`.
(cherry picked from commit 0af99e641a4329b57e48a314e2cedb592e02cd3b)