https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66212
--- Comment #2 from Jim Wilson <wilson at gcc dot gnu.org> --- libgcc should be built with debug info by default, but the one in /lib is probably stripped. Try setting LD_LIBRARY_PATH to point at the gcc-5.1 libraries that you built, instead of using the default ones provided by the OS in /lib.
