gnu: cross-gcc: Add a "lib" output.

gnu: cross-gcc: Add a "lib" output.

Add a "lib" output to cross-gcc. This requires an upstream GCC patch adding
support for --with-toolexeclibdir configure option. This option allows to
install cross-built GCC libraries in a specific location.

This also fixes the computation of TOOLDIR_BASE_PREFIX, that fails when
/gnu/store/... directories are involved.

* gnu/packages/patches/gcc-7-cross-toolexeclibdir.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/cross-base.scm (cross-gcc)[source]: Apply it,
[outputs]: add a "lib" output,
(cross-gcc-snippet): fix TOOLDIR_BASE_PREFIX.