The oe-core/poky change 'base.bbclass wipe ${S} before unpacking
source' (poky: a56fb90dc3805494eeaf04c60538425e8d52efc5, oe-core:
eccae514b71394ffaed8fc45dea7942152a334a1) wipes ${S} in do_unpack prior
to unpacking. This breaks our shared source as we set ${S} to the shared
location, and don't actually unpack anything (the result is we try to
build rustc without any source code, which fails predictably)
Avoid this by clearing do_unpack[cleandirs].
At the same time take pieces of gcc-shared-source.inc and note how & why
we differ from how gcc operates.
This is a bit of a hack, and only happens to work because we know the
exact method that do_unpack uses to clear ${S}, and using python()
happens to get called at the "right time".
487205331b