Bug 815931 - Fix up valgrind stack-scan annotations. r=terrence

Conditionalize on MOZ_VALGRIND instead of aliasing it to
JS_VALGRIND, and add includes for the memcheck macros.

The memcheck header is removed from jsgc.cpp, where is is
no longer necessary.

Based on a patch by Julian Seward.
Merge branch 'master' of https://github.com/mozilla/mozilla-central