This ensures that the ArgumentsAdaptorTrampoline does not clobber the
new.target value, but rather passes it through to the callee unaltered.
Note that callees do not yet use the new.target value so far.

This is a preparatory CL to allows us passing new.target in a register
instead of via a side-channel through the construct stub frame.

R=mvstanton@chromium.org
BUG=v8:4544
LOG=n

Review URL: https://codereview.chromium.org/1458103003

Cr-Commit-Position: refs/heads/master@{#32171}