Fix hwpmc sampling for MPC74xxx (G4) processors.

With this, hwpmc sampling now works correctly on these processors.

MFC after:	3 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275171 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f