[LIBC] Implement remainder of posix 'sched.h' minus `SCHED_SPORADIC`

Includes macros:
    linux/SCHED_OTHER // posix req
    linux/SCHED_FIFO // posix req
    linux/SCHED_RR // posix req
    linux/SCHED_BATCH
    linux/SCHED_ISO
    linux/SCHED_IDLE
    linux/SCHED_DEADLINE

Includes types:
    struct sched_param { int sched_priority; }

Includes functions:
    sched_setparam
    sched_getparam
    sched_setscheduler
    sched_getscheduler
    sched_get_priority_max
    sched_get_priority_min
    sched_rr_get_interval

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148069