POCL: reinterpret of a device array refers to undefined _reinterpret_exception #869

Calling reinterpret on a device array inside a kernel fails to compile on the POCL back-end (and therefore on CPU()):

using KernelAbstractions

@kernel function reint!(A, B)
    i = @index(Global)
    C = reinterpret(Int32, B)
    @inbounds A[i] = C[i]
end

B = KernelAbstractions.ones(CPU(), Float32, 4)
A = KernelAbstractions.zeros(CPU(), Int32, 4)
reint!(CPU())(A, B; ndrange=4)

ERROR: InvalidIRError: compiling MethodInstance for gpu_reint!(...) resulted in invalid LLVM IR
Reason: unsupported use of an undefined name (use of 'KernelAbstractions.POCL._reinterpret_exception')
Stacktrace:
 [1] reinterpret
   @ src/pocl/device/array.jl:273

src/pocl/device/array.jl was copied from OpenCL.jl in #556, where the check is GPUArrays._reinterpret_exception(T, a). KernelAbstractions doesn't depend on GPUArrays, so the call was left pointing at a function that doesn't exist. The fix is to carry the check (GPUArrays' src/host/base.jl) along with the device array code, and to test reinterpret in a kernel.

Found on main (1c2be35) while looking into #867.