powerpc: move get_pcpu() to machine/pcpu.h

[pkubaj]

Summary

Every other arch defines get_pcpu() in machine/pcpu.h, powerpc has it in
machine/cpufunc.h, which is entirely under _KERNEL. lsof includes
sys/systm.h without _KERNEL first on purpose, so on powerpc cpufunc.h
turns into an empty header and pcpup later expands to a call of an
undeclared get_pcpu(). Since sys/mount.h started pulling in sys/proc.h
this breaks the lsof build on powerpc64 and powerpc64le.