This passes the new.target value in a register instead of through a
side-channel via the construct stub. Note that only TurboFan code uses
the register value so far, but unoptimized code will be switched soon.

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

Review URL: https://codereview.chromium.org/1460503008

Cr-Commit-Position: refs/heads/master@{#32203}