llvm-readobj: zero out timestamp in COFF auto-generated test files.

The timestamp meant these files changed with each invocation of
relocs.py, confusing matters when we add relocations and need to
update the tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215350 91177308-0d34-0410-b5e6-96231b3b80d8