gnu: cross-base: Make `glibc-dynamic-linker' do the right thing.

* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Determine SYSTEM
  based on (%current-target-system) when it's true.
* gnu/packages/cross-base.scm (cross-gcc): Directly call
  (package-arguments gcc-4.7) from the body of `arguments'.