Use LWSYNC in place of SYNC/ISYNC in PPC spinlocks, where possible.

This is allegedly a win, at least on some PPC implementations, according
to the PPC ISA documents.  However, as with LWARX hints, some PPC
platforms give an illegal-instruction failure.  Use the same trick as
before of assuming that PPC64 platforms will accept it; we might need to
refine that based on experience, but there are other projects doing
likewise according to google.

I did not add an assembler compatibility test because LWSYNC has been
around much longer than hint bits, and it seems unlikely that any
toolchains currently in use don't recognize it.

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