[XRay] [compiler-rt] stop writing garbage in naive log records

Summary:
Turns out sizeof(packed) isn't as strong as we'd hoped. This makes sure
that when we initialize the padding, all 12 bytes will be zero.

Reviewers: dberris, kpw, eizan

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42494