systm.h: change pause from #define to inline function

There are drivers are using (*pause)(x, y) function pointers and depending
on how "pause" is used it gets replaced by pause_sbt causing compile time
failures.
Given "pause" is a generic enough name change it from a #define to an
inline function to avoid replacements where it should not.

MFC after: 2 weeks
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D36489