Issue description

Time for a libgcc package? 😁

We probably want to have libgcc_s matching our compiler (and not frozen to whatever is in our bootstrap tools).

diffoscope between libgcc_s.so.1 in stdenv.cc.cc.lib and glibc: https://gist.github.com/dtzWill/596b43d5c16178f570b5357e941ea587

Note that this libgcc_s (the old one from gcc 5.3 apparently) is what everything links to (that needs it), easy example is lzip.

A separate libgcc package (or compiler-rt? :D) would be best, but if nothing else we probably should use a libgcc that matches our compiler.

Steps to reproduce

$ nix build -f channel:nixos-unstable glibc
$ readelf -a ./result/lib/libgcc_s.so.1
...
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000e (SONAME)             Library soname: [libgcc_s.so.1]
 0x000000000000001d (RUNPATH)            Library runpath: [/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-glibc-2.23/lib]
...