[LinuxCNC/linuxcnc PR#295] uspace: Find top online CPU

NOTE This is PR relative to master branch, but we should consider whether to port it back to 2.7, because it affects that version. (however, it is not a regression)

Before this change, uspace assumed that online CPU numbers were consecutive, and attempted to schedule the real time tasks on the highest numbered CPU. For example, if SCNPROCESSORS_ONLN returns 8, then it is assumed that the online processors are 0-7, and processor number 7 is selected for real time tasks.

This is not always the case. For example, on a D525 with HT turned off via BIOS, it is reported that SCNPROCESSORS_ONLN is 2 and the online processors are 0,2. The old algorithm chooses processor number 1 for realtime, so no realtime thread is ever run.

While my HT system (i7-4790K) can turn off HT in the BIOS, it gets online processors 0-3 when this is done, so the buggy behavior is not provoked. However, if I take a CPU offline which is not the highest numbered CPU, e.g., with
$ sudo sh -c ‘echo 0 > /sys/devices/system/cpu/cpu6/online’
I can provoke the reported behavior. After this change, cpu number 7 is again selected (correctly) instead of cpu number 6 (incorrectly).

Closes: #294

Signed-off-by: Jeff Epler