gcc-shared-source: Fix source date epoch handling

The source date epoch for gcc isn't being transferred from the shared
workdir to the current WORKDIR for the specific recipe. This results in
the clamping code within sstate.bbclass using a value from 2011 which
changes the timestamps of many files. Since this happens part way
through the build, if pieces of gcc haven't built, or build/rebuild
later, we see things rebuilding when they should not and for generated
files, races are possible.

Fix this by copying the SDE from the shared workdir into the recipe
workdir.

[YOCTO #14953]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>