Makeconfig: Add libgcc directory to rtld-prefix search path

Makeconfig: Add libgcc directory to rtld-prefix search path

* This needs to be done twice, for test runs with and without
  --enable-hardcoded-path-in-tests
* Also, we need to query the used $(CC) for the library location.

* The container tests run ldd and dump the list of needed libraries, then
  copy these into the container.
* Without this patch, ldd may not find libgcc_s.so, resulting in"not found"
  output and no copying of the library.
* With this patch, the library is picked up independent of its location (as
  long as the proper directory is provided) and copied into the testroot.

* This does not mean yet that ld.so in the testroot actually finds it.

Signed-off-by: Andreas K. Hüttel <dilfridge@gentoo.org>