gcc: Rework shared work

The current implementation of shared work for gcc is at best confusing. It relies
on the fetch/unpack/patch tasks having exactly the same stamps and if this gets
broken for some reason, its hard to figure out what the problem is. It also
leads to complex code in bitbake.

The benefits of shared work for gcc are clear but a better approach is needed. This
patch adjusts things so that a single new recipe (gcc-source) provides the
fetch/unpack/patch/preconfigure tasks, the rest of gcc simply depends on these tasks
and have no fetch/unpack/patch tasks of their own.

This means we should get the significant benefits (disk usage/performance) of the
single source tree but in a way which has less potential for problems and is
easier for people to understand. The cost is an extra recipe/some inc files
which is probably a good tradeoff.

(From OE-Core rev: ceaa0a448dc5ebddb4f7fb94fb8a503a1c0248c3)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-shared-source.inc

@@ -0,0 +1,6 @@
+deltask do_fetch
+deltask do_unpack
+deltask do_patch
+
+do_configure[depends] += "gcc-source:do_preconfigure"
+do_populate_lic[depends] += "gcc-source:do_unpack"