#571 Be compatible with elftutils when handling SHT_NOBITS sections

[Joseph Koshy]

See the discussion titled "libelf and linux objtool" on -developers.

[Ed Maste]

Copied here to keep details in one place:

1. I also noticed that when elf_update recomputes the section layout,
   it leaves the section offset of SHT_NOBITS and SHT_NULL sections alone
   while the others are updated. elfutils sets the offset of the section
   based on the ending of the previous section. Does this offset have any
   meaning, or is it just arbitrary? Either way, maybe it makes sense to
   set it to the end of the previous section to avoid inheriting the
   value from the original section layout.