New scalar indexing error for `LinearAlgebra.tr` in Julia 1.12

[mtfishman]

For example:

julia> using JLArrays, LinearAlgebra

julia> tr(jl(randn(4, 4)))
ERROR: Scalar indexing is disallowed.
Invocation of getindex resulted in scalar indexing of a GPU array.
This is typically caused by calling an iterating implementation of a method.
Such implementations *do not* execute on the GPU, but very slowly on the CPU,
and therefore should be avoided.

If you want to allow scalar iteration, use `allowscalar` or `@allowscalar`
to enable scalar iteration globally or for the operations in question.
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:44
  [2] errorscalar(op::String)
    @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:151
  [3] _assertscalar(op::String, behavior::GPUArraysCore.ScalarIndexing)
    @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:124
  [4] assertscalar(op::String)
    @ GPUArraysCore ~/.julia/packages/GPUArraysCore/aNaXo/src/GPUArraysCore.jl:112
  [5] getindex
    @ ~/.julia/packages/GPUArrays/w335n/src/host/indexing.jl:50 [inlined]
  [6] #tr##0
    @ ./none:-1 [inlined]
  [7] MappingRF
    @ ./reduce.jl:92 [inlined]
  [8] _foldl_impl(op::Base.MappingRF{…}, init::Base._InitialValue, itr::StepRange{…})
    @ Base ./reduce.jl:50
  [9] foldl_impl
    @ ./reduce.jl:40 [inlined]
 [10] mapfoldl_impl
    @ ./reduce.jl:36 [inlined]
 [11] mapfoldl
    @ ./reduce.jl:167 [inlined]
 [12] mapreduce
    @ ./reduce.jl:299 [inlined]
 [13] reduce
    @ ./reduce.jl:479 [inlined]
 [14] tr(A::JLArray{Float64, 2})
    @ LinearAlgebra ~/.julia/juliaup/julia-1.12.0+0.aarch64.apple.darwin14/share/julia/stdlib/v1.12/LinearAlgebra/src/dense.jl:429
 [15] top-level scope
    @ REPL[5]:1
 [16] top-level scope
    @ REPL:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> versioninfo()
Julia Version 1.12.0
Commit b907bd0600f (2025-10-07 15:42 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, apple-m1)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 8 virtual cores)
Environment:
  JULIA_EDITOR = code

(jl_DlhBZE) pkg> st
Status `/private/var/folders/qz/q22pzwm144z9fq57mpf1hfp40000gq/T/jl_DlhBZE/Project.toml`
  [27aeb0d3] JLArrays v0.3.0
  [37e2e46d] LinearAlgebra v1.12.0

It looks like the issue is the introduction of this tr(::StridedMatrix) definition:
https://github.com/JuliaLang/LinearAlgebra.jl/blob/24f5e21cf3a560ca560c5a1759ff21ba68382ebd/src/dense.jl#L426-L430
while before GPU arrays were falling back to using this more generic tr(::AbstractMatrix) definition:
https://github.com/JuliaLang/LinearAlgebra.jl/blob/72d1a8fdb683fda7d019b2c365e87c2691476f80/src/generic.jl#L1021-L1024

A quick fix could be to define:

function LinearAlgebra.tr(A::AnyGPUMatrix)
    LinearAlgebra.checksquare(A)
    sum(diag(A))
end