This is a reland of 89737c5d.

Original change's description:
> PPC/s390: [turbofan] Ensure instruction start is in fixed register.
> 
> Port c462ddc8
> 
> Original Commit Message:
> 
>     This makes sure that {JSFunction} invocations always load the code start
>     address into the fixed {kJavaScriptCallCodeStartRegister} register. This
>     allows us to perform PC-relative operations more effective. For now this
>     only applies to code with {kCallJSFunction} linkage.
> 
> R=mstarzinger@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
> BUG=
> LOG=N
> 
> Change-Id: If346a3cbaea820b1fcec38c5105605496961a888
> Reviewed-on: https://chromium-review.googlesource.com/938721
> Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
> Cr-Commit-Position: refs/heads/master@{#51608}

Change-Id: I5b118c3903847cc13e2ce228e9713f8ae55ce193
Reviewed-on: https://chromium-review.googlesource.com/940342Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51619}