gcc: re-disable info documentation

The documentation of older GCC versions (E.G. 4.3.x) contain constructs
that are no longer accepted by recent (5.x) versions of makeinfo, breaking
the build.

The documentation isn't really needed, and takes up extra space/build time,
so work around it by disabling documentation, like we had before the
conversion of the toolchain to the package infrastructure.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>