libelf: Always update last_offset in updatefile and updatemmap.

libelf: Always update last_offset in updatefile and updatemmap.

When ELF section data was used, but not updated or marked as dirty and
there also existed non-dirty sections and some padding was needed between
the sections (possibly because of alignment) then elf_update might write
"fill" over some of the existing data. This happened because in that case
the last_position was not updated correctly.

Includes a new testcase fillfile that fails before this patch by showing
fill instead of the expected data in some section data. It succeeds with
this patch.

https://sourceware.org/bugzilla/show_bug.cgi?id=21199

Signed-off-by: Mark Wielaard <mark@klomp.org>