Despite the name, this package builds not only gcc, but also Newlib
(full and nano variants) all in the same go. The combined-tree build
procedure is documented at [1] and has a few advantages:
 * We can avoid the bootstrap problem of needing gcc to build Newlib
   and needing Newlib to build gcc. Cycles like these normally need
   to be broken with build profiles [2].
 * Newlib-nano is built as a second pass after the combined-tree build,
   using the just-built cross compiler.

[1] https://gcc.gnu.org/simtest-howto.html
[2] https://wiki.debian.org/DebianBootstrap
