package/libunwind: link to libgcc_s, not libgcc

Backport a fix from upstream to link against libgcc_s instead of libgcc.

Fixes ltrace build failures:
    http://autobuild.buildroot.net/results/a0c/a0c132f8cdea2c34ceff27606764a60c2853f644/
    http://autobuild.buildroot.net/results/67f/67fbd04d6ae187861fa1bf84d3df1877c7be6fb2/
    http://autobuild.buildroot.net/results/0ca/0ca564101b870e5953acb66ac832ce9e23198392/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>