RUNPATH validation

[Ludovic Courtès]

In ‘core-updates’ there’s a new ‘validate-runpath’ phase that checks all
the ELF binaries produced and fails if one of them has a dependency
(DT_NEEDED) that cannot be found in its RUNPATH.  It uses (guix elf) and
the new (guix build gremlin).

So now the “dangling .so references” bugs really need to be fixed.  ;-)
A few RUNPATH problems were fixed in ‘core-updates’, mostly related to
cross-compilation where binaries never had a valid RUNPATH due to the
lack of a cross ‘ld-wrapper’ to add ‘-rpath’ flags as we do natively.

Feedback welcome!

Ludo’.