Fix address computation for headers.

If the linker script has SECTIONS, the address computation is now
always done in LinkerScript::assignAddresses, like for any other
section.

Before fixHeaders would do a tentative computation that
assignAddresses would sometimes override.

This patch also splits the cases where assignAddresses needs to add
the headers to the first PT_LOAD and the address computation. The net
effect is that we no longer create an empty page for no reason in the
included test case, which matches bfd behavior.

llvm-svn: 287565