Custom build scripts should be able to set rustc flags

[catamorphism]

Related to #8522 but not quite the same: There needs to be a way, in librustpkg/api.rs, to set rustc flags. An example scenario, suggested by @erickt , is to make rustpkg build essentially run:

rustc --linker c++ --link-args $(llvm-config --ldflags --libs all) llvm.rs

[catamorphism]

#8522 will basically address this by exposing all of the rustc flags that rustpkg can set as part of the context, which the API already exposes. There are probably nicer ways.

[catamorphism]

I'm going to declare victory here, because the RustcFlags data type is exposed and once you've built a context (using a rustpkg API function), you can replace it with a RustcFlags of your own construction. I'm sure there are nicer ways, but I'll close this until someone has a concrete proposal for an improvement.