gnu: pkg-config: Memoize 'cross-pkg-config'.

When running:

  guix build --target=arm-linux-gnueabihf gdb -nd

this reduces the number of hits in the 'add-data-to-store' cache from
205 (25%) to 163 (21%).

* gnu/packages/pkg-config.scm (cross-pkg-config): Turn into an 'mlambda'.