Write a hash of the executable into the PE timestamp fields.

Windows tools treats the timestamp fields as sort of a build id,
using it to archive executables on a symbol server, as well as
for matching executables to PDBs.  We were writing 0 for these
fields, which would cause symbol servers to break as they are
indexed in the symbol server based on this value.

Although the field is called timestamp, it can really be any
value that is unique per build, so to support reproducible builds
we use a hash of the executable here.

Differential Revision: https://reviews.llvm.org/D43978

llvm-svn: 326920