glibc_multi: add output "static"

This change enables static compilation with glibc in a multilib setup. For
building a nix shell the output can now be referenced as follows:

devShells.default = pkgs.mkShell {
packages = [
pkgs.glibc_multi.static
];
};

In the implementation I was forced to make two design decisions:
1. The directory `$static/lib64` has to be a "real" directory and not a symlink.
Otherwise, the path to this directory is not added to $NIX_LDFLAGS, which in
turn causes the files to not be visible to gcc and ld during the build
process (for details see `pkgs/build-support/bintools-wrapper/setup-hook.sh`
line 16).
2. The directories `$static/lib` and `$static/lib64` have to contain symlinks
to both the files used for static and for dynamic linking (i.e. the outputs
of `static` and `out` of the 32 and 64 bit variant of glibc). Without this,
dynamic linking still works, however the resulting binaries will segfault.

pkgs/development/libraries/glibc/multi.nix

···
       "out"
       "bin"
       "dev"
-    ]; # TODO: no static version here (yet)
+      "static"
+    ];
     passthru = {
       libgcc = lib.lists.filter (x: x != null) [
         (glibc64.libgcc or null)
···
     cp -rs '${glibc32.dev}'/include "$dev/"
     chmod +w -R "$dev"
     cp -rsf '${glibc64.dev}'/include "$dev/"
+
+    mkdir -p "$static/lib" "$static/lib64"
+    # create symlinks for files used for dynamic linking
+    # -> removing this will cause dynamically linked programs to segfault
+    cp -rs '${glibc32.out}'/lib/* "$static/lib"
+    cp -rs '${glibc64.out}'/lib/* "$static/lib64"
+    # create symlinks for files used for static linking
+    cp -rs '${glibc32.static}'/lib/* "$static/lib"
+    cp -rs '${glibc64.static}'/lib/* "$static/lib64"
   ''