gnu: ld-wrapper-boot0: Work around strict evaluation of (%current-system).

gnu: ld-wrapper-boot0: Work around strict evaluation of (%current-system).

Reported by Mark H Weaver <mhw@netris.org>
Partly fixes <http://bugs.gnu.org/24832>.

'ld-wrapper-boot0' was evaluating strictly instead of lazily, leading to
invalid system types.

* gnu/packages/base.scm (make-ld-wrapper): Turn #:target into a
one-argument procedure.  Honor it.
* gnu/packages/commencement.scm (ld-wrapper-boot0): Fix 'name' argument
to 'make-ld-wrapper'.  Make #:target argument a procedure.
* gnu/packages/cross-base.scm (cross-gcc): Adjust #:target argument.