GCC’s libraries moved to a separate output

[Ludovic Courtès]

Commit 84e6756 in core-updates adds a “lib” output to GCC, meaning that
all its run-time support libraries (libgomp, libitm, etc.) go there,
while the rest goes to the main output.

The goal is to reduce disk usage for situations where only the libraries
are needed (for instance, in the installation image): the “lib” output
is around 17 MiB, and the rest is around 70 MiB.

It’s not causing any problems for the core packages, but we’ll see
whether Hydra uncovers something for higher-level packages.

Ludo’.