Fix pr18515.

My understanding (from reading just the llvm code) is that
* most ppc cpus have a "sync n" instruction and an msync alias that is "sync 0".
* "book e" cpus instead have a msync instruction and not the more
general "sync n"

This patch reflects that in the .td files, allowing a single codepath for
asm ond obj streamer and incidentelly fixes a crash when EmitRawText was
called on a obj streamer.