bug#34907: Go compiler refers to multiple versions of GCC

[Leo Famulari]

As seen in this example, the Go compilers are keeping references to both
GCC 5 and 6, which seems wrong:

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$ guix gc --references $(guix build --no-grafts go)        
/gnu/store/dna8kpb00kq176rz8x69yy4j33my2q55-perl-5.28.0
/gnu/store/5s2nib1lrd2101bbrivcl17kjx1mspw6-coreutils-8.30
/gnu/store/9mb1npi8xg8yi0swjf91p5n1qn3n315v-go-1.11.5
/gnu/store/dna8kpb00kq176rz8x69yy4j33my2q55-perl-5.28.0
/gnu/store/h90vnqw0nwd0hhm1l5dgxsdrigddfmq4-glibc-2.28
/gnu/store/izyk3kppj42pa8i2cq29bw3bnr1607ps-tzdata-2018i
/gnu/store/n2p1zs14y89lwkg9da68y12pc10c6sw9-gcc-5.5.0
/gnu/store/q19l04vd2za80mk1845pz7r8cz29qk43-bash-minimal-4.4.23
/gnu/store/sj8m05bfj2902h67c4qkmvnzg2pjdgsv-net-base-5.3
/gnu/store/ypiv8dj4lkvsnm82s639h18l87frrh5g-gcc-6.5.0-lib
/gnu/store/q19l04vd2za80mk1845pz7r8cz29qk43-bash-minimal-4.4.23
------

This is because libgcc 6 is specifically included in the Go
dependencies, but parts of GCC 5 come from gnu-build-system by default.

We should make the Go compilers use a single GCC version.