sched.h: Fix _S macros for better compatibility with glibc.

In e2650af157 was added "_S" macros for compatibility with glibc, but it's still
incompatible as under glibc the macros whose names end with "_S" operate on the
dynamically allocated CPU set(s) whose size is in bytes, not in bits.

While here remove limiting ifdef to non-kernel case.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38110
MFC after: 1 week