This reflects the new version numbering scheme for GCC 5 and up.
See <https://gcc.gnu.org/develop.html>.
* gnu/packages/gcc.scm (gcc-5.1): Rename to ...
(gcc-5): ... this.
(libstdc++-doc-5.1): Rename to ...
(libstdc++-doc-5): ... this, and use gcc-5.
* gnu/packages/commencement.scm (gcc-toolchain-5.1): Rename to ...
(gcc-toolchain-5): ... this, and use gcc-5.
* build-aux/hydra/gnu-system.scm (%core-packages): Use gcc-5.
