[VE] Transfer backend ownership

Kazushi Marukawa (kaz7) of NEC Solution Innovators will take over my
role as a code owner for the Vector Engine target. Erich Focht (efocht)
of NEC will assume the administrator role for the clang-ve-ninja
buildbot.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D121453