cargo-xbuild: Build script using std can't compile

I’m trying to use a build.rs file with the cc crate to compile C code as part of a library I’m using in my OS. This doesn’t seem to work when used with cargo-xbuild as the std crate doesn’t exist in the sysroot:

$ env RUST_TARGET_PATH="/home/user/code/rust/0x52a1/polymorphos/splatform/splatform_i686/" cargo xbuild --target=i686-unknown-polymorphos-elf

   Compiling libc v0.2.42
   Compiling dmm v0.1.0 (file:///home/user/code/rust/0x52a1/polymorphos/dmm)
error[E0463]: can't find crate for `std`=====================>         ] 32/38: libc, dmm                                                                                                                                                                                              
  |
  = note: the `i686-unknown-polymorphos-elf` target may not be installed

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: Could not compile `libc`.
warning: build failed, waiting for other jobs to finish...
error: build failed

Is there a way to make the build script compilation use the host’s target and default sysroot (so that the build script can actually do it’s job), and then use the target passed to cargo-xbuild and the newly built sysroot for the compilation of the crate itself?