move default pagelock implementation

Author: vchuravy

src/KernelAbstractions.jl

@@ -149,12 +149,13 @@ function copyto! end
 Pagelock (pin) a host memory buffer for a backend device. This may be necessary for [`copyto!`](@ref)
 to perform asynchronously w.r.t to the host/
 
+This function should return `nothing`; or `missing` if not implemented.
+
+
 !!! note
     Backends **may** implement this function.
 """
-function pagelock!(::Backend, x)
-    return nothing
-end
+function pagelock! end
 
 """
     synchronize(::Backend)
@@ -601,6 +602,10 @@ function functional(::Backend)
     return missing
 end
 
+function pagelock!(::Backend, x)
+    return missing
+end
+
 include("nditeration.jl")
 using .NDIteration
 import .NDIteration: get

src/cpu.jl

@@ -33,6 +33,7 @@ function copyto!(backend::CPU, A, B)
 end
 
 functional(::CPU) = true
+pagelock!(::CPU, x) = nothing
 
 function (obj::Kernel{CPU})(args...; ndrange = nothing, workgroupsize = nothing)
     ndrange, workgroupsize, iterspace, dynamic = launch_config(obj, ndrange, workgroupsize)