On S390, GCC seems to treat size_t as 'long_unsigned_int'. Which conflicts with the %d format specifier since that expects an 'int'. So changing the macro to PRIuS. This seems to fix the issue while retaining the expected behaviour for all other systems.

R=mlippautz@chromium.org, hpayer@chromium.org
BUG=none

Review-Url: https://codereview.chromium.org/2759963003
Cr-Commit-Position: refs/heads/master@{#43988}