[libc] Fix sched_get_priority_max() syscall invocation

Previously used SYS_sched_get_priority_min. This also updates the tests
for SCHED_RR and SCHED_FIFO to check max_priority > min_priority, rather
than >= in order to catch future breakages.

Reviewed By: michaelrj, goldstein.w.n

Differential Revision: https://reviews.llvm.org/D149102