cc-wrapper: when merging gcc32 and gcc64, merge libgcc as well

Our gcc_multi and glibc_multi expressions merge together a
32-bit-targeted and 64-bit-targeted gcc. However they do not thread
through the passthru.libgcc from these merged gccs.

This commit corrects that.

It also extends passthru.libgcc to allow a *list* rather than just a
single outpath.

Resolves part of #221891 (at least getting it back to the error
message it gave before).

pkgs/build-support/cc-wrapper/default.nix

···
     ''
     # this ensures that when clang passes -lgcc_s to lld (as it does
     # when building e.g. firefox), lld is able to find libgcc_s.so
-    + lib.optionalString (gccForLibs?libgcc) ''
-      echo "-L${gccForLibs.libgcc}/lib" >> $out/nix-support/cc-ldflags
-    '')
+    + concatMapStrings (libgcc: ''
+      echo "-L${libgcc}/lib" >> $out/nix-support/cc-ldflags
+    '') (lib.toList (gccForLibs.libgcc or [])))
 
     ##
     ## General libc support

pkgs/development/compilers/llvm/multi.nix

···
       libc = gcc_multi_sysroot;
     };
 
-    gccForLibs = gcc_multi_sysroot;
+    gccForLibs = gcc_multi_sysroot // { inherit (glibc_multi) libgcc; };
   };
 
 in clangMulti

pkgs/development/libraries/glibc/multi.nix

···
-{ runCommand, glibc, glibc32
+{ lib
+, runCommand, glibc, glibc32
 }:
 
 let
···
 in
 runCommand "${nameVersion.name}-multi-${nameVersion.version}"
   # out as the first output is an exception exclusive to glibc
-  { outputs = [ "out" "bin" "dev" ]; } # TODO: no static version here (yet)
+  {
+    outputs = [ "out" "bin" "dev" ];  # TODO: no static version here (yet)
+    passthru = {
+      libgcc = lib.lists.filter (x: x!=null) [
+        (glibc64.libgcc or null)
+        (glibc32.libgcc or null)
+      ];
+    };
+  }
   ''
     mkdir -p "$out/lib"
     ln -s '${glibc64.out}'/lib/* "$out/lib"