mdb_v8: update for v0.12

Bugs fixed:

* v0.12 and later: in-object properties not printing correctly.
* 64-bit: not printing external strings correctly (offset was hardcoded
for 32-bit). This would happen with "::jsstack -vn0" because the
script "node.js" wasn't printed correctly, at least with 0.10 core
files.
* 64-bit: printing JS source (via "::jsstack -v") emits errors and shows
the wrong code.
* Several build warnings.
* Two-byte strings are unnecessarily truncated.
* Could print friendlier note when given obviously bogus function token
positions.

New features:

* ::jsstack prints much cleaner output by default.
* ::jsprint keys are now quoted.
* ::jsstack -v includes "this" value for each function on the stack.
* ::jsstack -v includes more details about each argument (constructor
names for each object).
* new commands: ::jsconstructor, ::jsfunctions, ::jssource, ::nodebuffer
and ::v8internal.
* ::findjsobjects and ::jsprint hidden flags for developers to measure
and improve test coverage.
* internal jsobj_properties() function is much better documented.

Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>