Layout one section until no relaxations are done and then move to the next

Layout one section until no relaxations are done and then move to the next
section.

This helps because in practice sections form a dag with debug sections pointing
to text sections. Finishing up the text sections first makes the debug section
relaxation trivial.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122314 91177308-0d34-0410-b5e6-96231b3b80d8