It becomes a library of some sort, so that multiple scripts can benefit
from it to build different versions of GCC.
The GPG key associated with GCC is also refreshed from keys.gnupg.net,
adding a new subkey, used to sign newer versions of GCC (and
postprocessed with pgpstrip to make it smaller).
We're soon going to build multiple versions of clang and gcc for linux,
and we need to differentiate them. Furthermore, there is a need for the
base-toolchains builds to use a fixed version of clang and gcc. So
rename the clang and gcc toolchain jobs to include their version, add
aliases to satisfy all existing jobs, and adjust the base-toolchains
jobs to use the explicit version.
