Add rustc_flags to rust_doc_test

[rdelfin]

Due to a quirk in our toolchain, we need to pass in -Crelocation-model=static to a set of executable binaries when building them to let them work (see #118 for more details). As a result, when running a rust_doc_test, this flag needs to be used during compilation. However, there's currently no way of setting this on those targets. The only work around is to skip execution of the test with the no_run directive, which is not ideal

[illicitonion]

AFAIK rust_doc_test takes a pointer to an existing target, and all of the targets you can point at allow you to specify rustc_flags - are you setting on the other target, and is it getting dropped or something?

Can you put together a small example to show what you're trying to do?