[libc] Correct standard for getcpu

Given this is a syscall wrapper and not in POSIX, this should actually
be defined under the Linux standard.

https://man7.org/linux/man-pages/man2/getcpu.2.html confirms.

Author: boomanaiden154

libc/include/sched.yaml

@@ -20,7 +20,7 @@ functions:
       - type: const cpu_set_t *
   - name: getcpu
     standards:
-      - POSIX
+      - Linux
     return_type: int
     arguments:
       - type: unsigned int *