Warn when a kernel has many arguments (#555)

* Warn when a kernel has many arguments

* count and check arg sizes

* check for all context devices

* refactor the size checking code and adjust pocl's limits

* cleanup comments

* Reword GPU warning

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Only warn when approaching the limit if param size is estimate

Co-authored-by: Andreas Klöckner <inform@tiker.net>