[ELF] - Move section factory out from writer to make it reusable.

Since linkerscript should create sections by itself
(if SECTIONS command is present),
then we might want to reuse the OutputSectionFactory (D19976 already do that now),
so this patch moves it out from writer cpp file for that purpose.

Differential revision: http://reviews.llvm.org/D19977

llvm-svn: 275161