Sort sections in relocatable files.

The final section order in relocatable files was just a side effect
of the atom sorter.  This meant that sections like __data were before
__text because __data has RW permissions and __text RX and RW was less
than RX in our enum.

Final linked images had an actual section/segment sorter.  There was no
reason for the difference, so simplify a bunch of code and just use the
same sorted for everything.

Reviewed by Lang Hames.

http://reviews.llvm.org/D15868

llvm-svn: 256786