Don't throw unconditionally when matrix is singular

[andreasnoack]

Following up on https://github.com/JuliaGPU/CuArrays.jl/issues/107#issuecomment-414659174. The base behavior is now

julia> lu(zeros(2,2), check = true)
ERROR: SingularException(1)
Stacktrace:
 [1] checknonsingular at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/LinearAlgebra/src/factorization.jl:12 [inlined]
 [2] #lu!#97(::Bool, ::Function, ::Array{Float64,2}, ::Val{true}) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/LinearAlgebra/src/lu.jl:41
 [3] #lu! at ./none:0 [inlined]
 [4] #lu#101 at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/LinearAlgebra/src/lu.jl:142 [inlined]
 [5] #lu at ./none:0 [inlined] (repeats 2 times)
 [6] top-level scope at none:0

julia> lu(zeros(2,2), check = false)
Failed factorization of type LU{Float64,Array{Float64,2}}