init: only use signed-integer-overflow sanitizer

We've blown up twice in init due to the unsigned integer overflow
sanitizer despite the overflows in question being both defined and
intentional.

Bug: 63680332
Test: boot
Change-Id: I08effe3202ac1367d858982ff5478b3a088bab37
(cherry picked from commit 2ffd65e1d12940b5e13ea743bb88e94a57e7ca97)
2 files changed