Auto merge of #2384 - youknowone:apple-sched, r=JohnTitor

Add SCHED_* consts to apple platforms

- MacOS: https://github.com/phracker/MacOSX-SDKs/blob/11.3/MacOSX10.11.sdk/usr/include/pthread/pthread_impl.h#L50-L52

I didn't find online references but checked also other SDKs in local machines.

```shell
$ rg '#define SCHED_'
MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

AppleTVOS.platform/Developer/SDKs/AppleTVOS.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

AppleTVSimulator.platform/Developer/SDKs/AppleTVSimulator.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

WatchOS.platform/Developer/SDKs/WatchOS.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

WatchSimulator.platform/Developer/SDKs/WatchSimulator.sdk/usr/include/pthread/pthread_impl.h
54:#define SCHED_OTHER                1
55:#define SCHED_FIFO                 4
56:#define SCHED_RR                   2

```

Author: bors

libc-test/semver/apple.txt

@@ -1108,6 +1108,9 @@ SAE_ASSOCID_ANY
 SAE_CONNID_ALL
 SAE_CONNID_ANY
 SCALE_PPM
+SCHED_OTHER
+SCHED_FIFO
+SCHED_RR
 SCM_CREDS
 SCM_RIGHTS
 SCM_TIMESTAMP

src/unix/bsd/apple/mod.rs

@@ -3471,6 +3471,10 @@ pub const FD_SETSIZE: usize = 1024;
 
 pub const ST_NOSUID: ::c_ulong = 2;
 
+pub const SCHED_OTHER: ::c_int = 1;
+pub const SCHED_FIFO: ::c_int = 4;
+pub const SCHED_RR: ::c_int = 2;
+
 pub const EVFILT_READ: i16 = -1;
 pub const EVFILT_WRITE: i16 = -2;
 pub const EVFILT_AIO: i16 = -3;