Build the GPU lu_instance directly instead of calling lu

[singhharsh1708]

Fixes #501.

lu_instance for GPU matrices builds the instance by calling lu on an adapted 0x0 array,
which only works for backends that define their own lu. JLArrays does not, so it falls
through to LinearAlgebra's generic path and dies:

using JLArrays, ArrayInterface
ArrayInterface.lu_instance(jl(ones(3, 3)))
# ERROR: Illegal conversion of a JLArray to a Ptr

There is no size that works, lu(jl(ones(0,0))) hits LAPACK getrf! and lu(jl(ones(3,3)))
hits "Scalar indexing is disallowed", so routing through lu is the problem rather than the
0x0.

This builds the LU directly, the same way the Matrix, Symmetric and
Tridiagonal/Diagonal/SymTridiagonal methods already do in ArrayInterface.jl, so no
backend lu is needed. After:

ArrayInterface.lu_instance(jl(ones(3, 3)))
# LU{Float64, JLArray{Float64, 2}, JLArray{Int64, 1}}

which mirrors the CPU result LU{Float64, Matrix{Float64}, Vector{Int64}}.

It matters downstream because JLArrays is what packages use to exercise GPU code paths without
a GPU. OrdinaryDiffEq's GPU autodiff test is failing through this on Julia 1, lts and pre right
now, and #467 reported the same shape for Metal.

Worth checking before merge: I only have JLArrays here, no CUDA/AMDGPU/Metal, so I could not
confirm the ipiv element type matches what those backends' own lu returns. The CPU methods
all use BlasInt and I followed them. If a backend's lu returns Int32 there, this changes
the instance type for it.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.93%. Comparing base (6950d2a) to head (7400d47).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/ArrayInterfaceGPUArraysCoreExt.jl	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   61.32%   60.93%   -0.40%     
==========================================
  Files          15       15              
  Lines         618      622       +4     
==========================================
  Hits          379      379              
- Misses        239      243       +4     

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