Switch CallConstructStub to take new.target in register.

Switch CallConstructStub to take new.target in register.

This changes the calling convention of the CallConstructStub to take
the original constructor (i.e. new.target in JS-speak) in a register
instead of magically via the operand stack. For optimizing compilers
the operand stack doesn't exist, hence cannot be peeked into.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1237813002

Cr-Commit-Position: refs/heads/master@{#29681}