DRTVWR-418, MAINT-6996: Update Mac LLMemory::getCurrentRSS().

Evidently the Mac implementation of LLMemory::getCurrentRSS() goes back to
OS X 10.3, because there was a helpful comment of the form:

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The API used here is not capable of dealing with 64-bit memory sizes, but is
available before 10.4.

Once we start requiring 10.4, we can use the updated API, which looks like
this:

[new current implementation]

Of course, this doesn't gain us anything unless we start building the viewer
as a 64-bit executable, since that's the only way for our memory allocation to
exceed 2^32.
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Hey, guess what, we're building 64-bit viewers now!

Thank you, whoever thoughtfully noted that, both for calling out the issue and
sparing us the research. (The comment goes back to Subversion days, so hg
blame shows only the merge-to-release changeset.)