JNI direct buffer function speedup, part 2.

JNI direct buffer function speedup, part 2.

This converts the three direct buffer functions from JNI to internal VM
calls.  As a bonus, we grab PlatformAddress.osaddr directly instead of
retrieving it with PlatformAddress.toLong().

We're still calling through getEffectiveAddress(), which is where most
of the complexity lies.

Nudged a couple of comments.