Minor refactoring of ElfBuilder.

 * Nest section builder classes within the main builder
   class to reduce verbosity caused by templates.
 * Move .eh_frame and .eh_frame_hdr section headers earlier
   so that the section headers are sorted by file offset.
 * Do not emit PT_NULL header if .eh_frame_hdr is missing.

None of these changes should have any material impact.
This CL just prepares ground for the following CL.

Change-Id: I56f9825892adb01d413a18ca10cb5c805cb8cd78