pkgsStatic: Pass hostPlatform.gcc attribute

To build the security wrappers[1] the pkgsStatic stdenv is used, so the
binaries are static. However, the hostPlatform may have gcc attributes
that are *required* to build binaries so they can run on the host
platform. In particular, this is the case when using gcc.arch, which
ends up injecting -march=... in the gcc wrapper. Those attributes are
not contained in hostPlatform.parsed.

This change sets the same gcc attributes found in the hostPlatform for
the pkgsStatic cross system, so it can build binaries with the same gcc
flags.

[1]: nixos/modules/security/wrappers/default.nix

pkgs/top-level/stage.nix

···
           if stdenv.isLinux
           then makeMuslParsedPlatform stdenv.hostPlatform.parsed
           else stdenv.hostPlatform.parsed;
-      } // lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") {
-        gcc.abi = "elfv2";
+        gcc = lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") { abi = "elfv2"; } //
+          stdenv.hostPlatform.gcc or {};
       };
     });
   };