[patch] Quick try at fixing pr18515

[Rafael Espíndola]

I got curious as to what was needed to fix pr18515 and decided to try
to patch it. This patch is under the following assumptions I got from
reading the existing code

* 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"

What this patch does is predicate the sync instruction and its aliases
by IsNotBookE and then add a MSYNC instruction predicated on IsBookE.

Cheers,
Rafael
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