ELF2: Make singleton output sections globally accessible.

Previously, output sections that are handled specially by the linker
(e.g. PLT or GOT) were created by Writer and passed to other classes
that need them. The problem was that because these special sections
are required by so many classes, the plumbing work became too much
burden.

This patch is to simply make them accessible from anywhere in the
linker to eliminate the plumbing work once and for all.

http://reviews.llvm.org/D13486

llvm-svn: 249590