Compute jl_array_len from the dims for N-d arrays (#62813)
For arrays with more than one dimension `jl_array_len` returned the
length of the backing `Memory`. However, N-d arrays with excess memory
capacity, and even a nonzero memoryref offset, can be created by
`reshape`ing a vector with spare capacity from e.g. `sizehint!` or
`push!`/`popfirst!`. This made every C code path that uses
`jl_array_len` observe the wrong length. Splatting such an array (the
array fast path of `jl_f__apply_iterate`) and `Core.svec` produced
extra, potentially uninitialized, elements, and `jl_array_copy` copied
the capacity rather than the length, reading past the end of the
`Memory` when the offset is nonzero.
Make `jl_array_len` compute the product of the dimensions instead.
Assisted-by: Claude Code (Fable 5)
