Revert of PPC/s390: Internalize strings in-place (reland) (patchset #1 id:1 of https://codereview.chromium.org/2628643002/ )

Reason for revert:
Original CL https://codereview.chromium.org/2626893005 got reverted.

Original issue's description:
> PPC/s390: Internalize strings in-place (reland)
>
> Port 4c699e34
>
> Original Commit Message:
>
>     using newly introduced ThinStrings, which store a pointer to the actual,
>     internalized string they represent.
>
>     (Previously landed as #42168 / af51befe.
>
> R=jkummerow@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
> BUG=v8:4520
> LOG=N
>
> Review-Url: https://codereview.chromium.org/2628643002
> Cr-Commit-Position: refs/heads/master@{#42202}
> Committed: https://chromium.googlesource.com/v8/v8/+/aa0cb2eaf830e03fced1e00134fb65938df02388

TBR=jkummerow@chromium.org,joransiu@ca.ibm.com,jyan@ca.ibm.com,michael_dawson@ca.ibm.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:4520

Review-Url: https://codereview.chromium.org/2628923004
Cr-Commit-Position: refs/heads/master@{#42304}