sstate: Fix shared work preconfigure task handling

When the preconfigure task was added to gcc, it wasn't added to sstate's list
of possible shared work tasks. This meant that diffsigs wasn't able to find
the preconfigure sigdata/info file since it has inconsistent naming.

This adds the task name to the list. Ideally this list would be autogenerated
or not even required, right now its a sanity test that the shared work
code works as intended so is best left as is.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
sstatesig: Add try/except around the stat calls

Its possible sstate symlinks to other sstate mirrors which then my get
removed/cleaned. If we find invalid symlinks, skip over them rather
than error with a backtrace.

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