Improve clGetPlatformInfo
Improve clGetPlatformInfo

* fix reported OpenCL Version
  From Khronos doc:
    OpenCL version string. Returns the OpenCL version supported by the
    implementation. This version string has the following format:
    OpenCL<space><major_version.minor_version><space><platform-specific information>
* all supported extensions should declared here so that programs can
  found them through an ICD Loader (ie installed headers files on the
  system will be the ones from Khronos, not the one provided by Pocl
  when multiple OCL ICD are available)
* use POCL as suffix extension. Pocl does not provide private
  extensions (yet), but we can declare a raisonable extension suffix
  (ie POCL)

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