build: bring back libgcc_s.so

Commit c9e61d4a ("build: link libstdc++, libgcc_s only once") threw
away libgcc_s.so since we already link with libgcc.a and libgcc_eh.a, which
provide the same symbols, and since having the same symbols in multiple
objects violates certain C++ rules.

However, libgcc_eh.a provides certain symbols only as local symbols, which
means they aren't available to the payload.  This manifests itself in errors
such as failing to find _Unwind_Resume if an exception is thrown.

(This is likely due to the requirement that mulitple objects linked with
libgcc_eh.a work together, which also brings some confidence that the ODR
violations of having two versions of the library won't bite us).

Fix the problem by adding libgcc_s.so to the filesystem and allowing
the payload to link to it.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>