Fixes a bug exposed with be5ec2ebb4bde4b4f702a30f08849dff05cceccd, which
introduces the “avr” target, which has no hyphen.
* gnu/packages/gnupg.scm (libgpg-error)[arguments]: In ‘cross-symlinks’
phase, adjust code to deal with TARGET lacking a hyphen.
Change-Id: I1aff6a94a7c92626f931c459d1dc0509f9bcfc00