gnu: gettext, attr, acl: Disable `check' phase twiddling when cross building.

* gnu/packages/gettext.scm (gettext)[arguments]: Disable `check' phase
  manipulations when (%current-target-system) is true.
* gnu/packages/attr.scm (attr)[arguments]: Likewise.
* gnu/packages/acl.scm (acl)[arguments]: Likewise.