white-list .rcgu.o files in the linking phase, otherwise dump them into the normal linking pile

[jkelleyrtp]

Attempts to fix #4237 by white-listing just rust's incremental object files and instead lets the linker deal with "funny" rlibs. Note that for rlibs that contain "funny" artifacts and rust artifacts, we still submit the rust ones.

[gmorenz]

This is clever! I'll try testing it locally.

[gmorenz]

This works great locally.

Are there cases where editing non-ffi rust breaks with this? The possibility that comes to mind is that if a generic function calls an FFI function, will the linker include the object file needed for that FFI function in v0 before there are any calls to it?

[jkelleyrtp]

Are there cases where editing non-ffi rust breaks with this? The possibility that comes to mind is that if a generic function calls an FFI function, will the linker include the object file needed for that FFI function in v0 before there are any calls to it?

Ended up testing it and seemed to work out fine, but the -L flag needs to be set before you add the extern block. Ie if you want to reference libc externs, they need to be available in your build.rs first.