Also changing the wording of comment under s390
instruction-selector to match PPC.

Change-Id: I8fa77cbf51872792acd0e89915cb11d4759d51f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2305850Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#68948}