Restrict lwsync to powerpc64 only

Nearly all 32-bit powerpc hardware treats lwsync as sync, and some cores
(Freescale e500) trap lwsync as an illegal instruction, which then gets
emulated in the kernel.  To avoid unnecessary traps on the e500, use
sync on all 32-bit powerpc.  This pessimizes 32-bit software running on
64-bit hardware, but those numbers should be slim.