[PECOFF] Add a generic section writer.

Instead of having multiple SectionChunks for each section (.text, .data,
.rdata and .bss), we could have one chunk writer that can emit any sections.
This patch does that -- removing all section-sepcific chunk writers and
replace them with one "generic" writer.

This change should simplify the code because it eliminates similar-but-
slightly-different classes.

It also fixes an issue in the previous design. Before this patch, we could
emit only limited set of sections (i.e. .text, .data, .rdata and .bss). With
this patch, we can emit any sections.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195797 91177308-0d34-0410-b5e6-96231b3b80d8