Major refactor of the ELFWriter code. Instead of building up one big

vector that represents the .o file at once, build up a vector for each
section of the .o file.  This is needed because the .o file writer needs
to be able to switch between sections as it emits them (e.g. switch
between the .text section and the .rel section when emitting code).

This patch has no functionality change.

llvm-svn: 22453