[COFF] Allow embedded directives to be separated by null bytes

The PE spec says that they will be separated by spaces, but link.exe
handles it just fine if they are separated by null bytes as well.

This adds tests to the lld repo, with the actual functional change
in LLVM in SVN r342204.

Differential Revision: https://reviews.llvm.org/D52014

llvm-svn: 342206