libunwind, which the next patches will use to implement a more reliable
backtrace(), needs the msync() function. It doesn't need it to actually
sync anything - just to recognize valid frame addresses (stacks are
always mmap()ed).

Note this implementation does the checking, but is missing the "sync" part
of msync ;-) It doesn't matter because:

1. libunwind doesn't need (or want) this syncing, and neither does anything
   else in the Java stack (until now, msync() was never used).

2. We don't (yet?) have write-back of mmap'ed memory anyway, so there's
   no sense in doing any writing in msync either. We'll need to work on
   a full read-write implementation of file-backed mmap() later.