Support building elfutils against musl

Elfutils uses argp, fts and obstack glibc extensions, none of which are
provided by musl.

Avoid compiling sources that use argp, they are only needed by eu-*
tools that we don't compile.

Use bionic's implementation of fts by linking libfts.

Use a copy of obstack from libiberty from external/obstack.

Bug: 190084016
Test: m USE_HOST_MUSL=true adbdw abidiff abitidy
Test: ./post_update.sh
Change-Id: I34dcac4110db0c13f7e1520c92844c45b95abfa0
5 files changed