Blender Git Loki

Threads: Cache result of syscall when querying number of system threads

Number of system threads is quite difficult to change without need of blender
restart, so we can cache result of the systcalls (which are not really cheap)
in order to be able to call BLI_system_thread_count() without worrying of
performance issues in that function.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D1342