New patch (relative to master) attached.  Result:

...
   Compiling bootstrap v0.0.0 (file:///tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/src/bootstrap)
...
     Running `/gnu/store/vqqyfd20cci8ngffx8ip30xj1zghj3d3-rustc-bootstrap-1.20.0/bin/rustc --crate-name rustdoc src/bootstrap/bin/rustdoc.rs --crate-type bin --emit=dep-info,link -C debug-assertions=off -C overflow-checks=on -C metadata=be82d62669cb1c84 -C extra-filename=-be82d62669cb1c84 --out-dir /tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps -L dependency=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps --extern serde=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libserde-632d9b22ceaaf392.rlib --extern getopts=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libgetopts-3bd92ee3d46aec42.rlib --extern filetime=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libfiletime-c92abda52111c8f6.rlib --extern lazy_static=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/liblazy_static-2aedc8916caa605d.rlib --ex
 tern cmake=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libcmake-cb4e4459aab5227f.rlib --extern build_helper=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libbuild_helper-a3d8a479aaa255b5.rlib --extern toml=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libtoml-9a87f4f0395599e4.rlib --extern gcc=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libgcc-f472059c4232400f.rlib --extern serde_derive=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libserde_derive-46fcba9d518ff59d.so --extern serde_json=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libserde_json-d578450b81b0fe32.rlib --extern num_cpus=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libnum_cpus-c524032e97e252a3.rlib --extern libc=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/de
 ps/liblibc-785194da5730896d.rlib --extern bootstrap=/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/deps/libbootstrap-5489136beef8bcf0.rlib`
    Finished dev [unoptimized] target(s) in 95.84 secs
running: /tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/bootstrap build -v
/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/bootstrap: error while loading shared libraries: libgcc_s.so.1: cannot open shared object file: No such file or directory
Traceback (most recent call last):
  File "/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/src/bootstrap/bootstrap.py", line 816, in <module>
    main()
  File "/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/src/bootstrap/bootstrap.py", line 799, in main
    bootstrap()
  File "/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/src/bootstrap/bootstrap.py", line 790, in bootstrap
    run(args, env=env, verbose=build.verbose)
  File "/tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/src/bootstrap/bootstrap.py", line 148, in run
    raise RuntimeError(err)
RuntimeError: failed to run: /tmp/guix-build-rustc-1.21.0.drv-0/rustc-1.21.0-src/build/bootstrap/debug/bootstrap build -v
make: *** [Makefile:24: all] Error 1
phase `build' failed after 96.3 seconds