Modify ppc64/s390x maintainer list

Some ppc64/s390x maintainers are moving to IBM.

Change-Id: I775d0e84d97e24f383fb2682e7a3f96097cdb77b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6700140
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Junliang Yan <junyan1@ibm.com>
Cr-Commit-Position: refs/heads/main@{#101238}

PPC_OWNERS

@@ -1,4 +1,4 @@
-junyan@redhat.com
+junyan1@ibm.com
 joransiu@ca.ibm.com
 midawson@redhat.com
-mfarazma@redhat.com
+mfarazma@ibm.com

S390_OWNERS

@@ -1,4 +1,4 @@
-junyan@redhat.com
+junyan1@ibm.com
 joransiu@ca.ibm.com
 midawson@redhat.com
-mfarazma@redhat.com
+mfarazma@ibm.com