X.py overwrites custom stdarch submodule

Hi,

I've added a new arch (one of the experimental targets of LLVM) and a new target to rustc, and it seems to be working when cross-compiling for some trivial code which uses asm!.

Now I want to define some intrinsics for my arch in core_arch and import them to my code from core::arch. I thought I'd fork the stdarch, create a branch for my changes, and then change the library/stdarch git submodule and rebuild libcore with x.py. But every time I try to rebuild it, x.py overwrites my changes (it fetches stdarch from my fork but instead of checking out my branch it apparently keeps using the master which I haven't changed).

In the end, I just need cargo to use my custom cross-compiled libcore reexporting my own version of stdarch, what's the simplest way of doing this?

Thanks!

build.submodules in config.toml indicates whether they should be managed by x.