gcc: Put runtime libraries in a separate output

GCC provides a number of libraries that are used by programs built by
GCC, in particular libgcc_s.so and libstdc++.so.  This caused programs
that used these libraries to have a runtime dependency on all of GCC
(~77 MiB).  Now they only depend on the "lib" output of GCC (~1.6
MiB).

With this and previous multiple-output improvements, closure sizes are
reduced a lot:

hello:       41 MiB -> 22 MiB
patchelf:   118 MiB -> 23 MiB
pan:        364 MiB -> 90 MiB