Don't depend on lld creating space for the headers.

Currently lld will implicitly reserve space for the headers. This is
not the case is bfd, where it is the script responsibility to use
SIZEOF_HEADERS. This means that a script not using SIZEOF_HEADERS and
expecting the address of the first section to be 0 would fail with lld.

I am fixing that is the next commit. This one just makes the tests
explicitly use SIZEOF_HEADERS to avoid the dependency on the current
behaviour.

llvm-svn: 282814