When calling into C++ for a ConstructStub, we need to enter the target
context manually currently, which seems to be too fragile and easy to
forget. So instead of doing that manually, we just always enter the
correct context in the trampoline.

Drive-by-fix: Trivial cleanups for some builtins.

R=cbruni@chromium.org

Review URL: https://codereview.chromium.org/1551473002

Cr-Commit-Position: refs/heads/master@{#33051}