glsl/builtin_functions: Rename int64 function to int64_avail

* int64 is a core type on Haiku (and potentially other platforms)
* rename to int64_avail matching other similar calls

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
91 jobs for !8266 with dec2020-haiku-fixes in 19 minutes and 43 seconds (queued for 6 seconds)
merge request