Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Move helper functions `_libelf_ar_get_{name,number,string}()` and
`_libelf_ar_open()` to a new compilation unit "libelf_ar_util.c"
to break the circular dependency between "elf_memory.o" and
"libelf_ar.o".

Obtained from: elftoolchain
MFC after: 1 month