auto merge of #11742 : eminence/rust/dylib-test-fix, r=alexcrichton

On my Gentoo Linux machine, the c-dynamic-dylib test is failing, because libcfoo can't be found.  bar has a correct rpath for finding libfoo.so, but libfoo.so's rpath doesn't contain the right entries for finding libcfoo.

Below is the test failure on my machine.  This test pass with this commit:

```
maketest: c-dynamic-dylib
----- /storage/home/achin/devel/rust/src/test/run-make/c-dynamic-dylib/ --------------------
------ stdout ---------------------------------------------
make[1]: Entering directory `/storage/home/achin/devel/rust/src/test/run-make/c-dynamic-dylib'
gcc -Wall -Werror -g -fPIC -m64 -L /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib -c -o /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/libcfoo.o cfoo.c
gcc -Wall -Werror -g -fPIC -m64 -L /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib -o /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/libcfoo.so /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/libcfoo.o -shared
/storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/stage2/bin/rustc --out-dir /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib -L /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib foo.rs
/storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/stage2/bin/rustc --out-dir /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib -L /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib bar.rs
/storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/bar
rm /storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/libcfoo.o
make[1]: Leaving directory `/storage/home/achin/devel/rust/src/test/run-make/c-dynamic-dylib'

------ stderr ---------------------------------------------
/storage/home/achin/devel/rust/x86_64-unknown-linux-gnu/test/run-make/c-dynamic-dylib/bar: error while loading shared libraries: libcfoo.so: cannot open shared object file: No such file or directory
make[1]: *** [all] Error 127

------        ---------------------------------------------

```