Define AtomicWord as Atomic32 on AIX.
Test _ARCH_PPC on AIX.
Undef UNKNOWN, FALSE and TRUE leaked into public identifier space
by AIX header.
Define AtomicWord as Atomic32 on AIX.
Configure additional platform sources for AIX.
Add AIX to GuessOS and ppc for GuessArchitecture.
Add AIX to OSes that need d8-posix.cc.