[gdb] Define print functions used by gdb macros in the top level namespace to make them always be available.

This should fix the following errors that sometimes appear when using V8 gdb macros:
  Cannot evaluate function -- may be inlined
or
  No symbol "xxx" in namespace "v8::internal".

Review-Url: https://codereview.chromium.org/2186293002
Cr-Commit-Position: refs/heads/master@{#38145}