icecc.bbclass: Move to shared work directory

Generate the icecc toolchains in a shared work directory. This class was
already setup to correctly synchronize creating the toolchains in a
shared location before the RSS changes, so return to that behavior
instead of generated the toolchains in each recipe's sysroot.
Additionally, it makes no sense for each recipe to generate a toolchain,
only to find it was already generated and uploaded to the compile server
by another recipe.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>