rust i386 on amd64

[Mike Pumford <mpumford%mudcovered.org.uk@localhost>]

I'm trying to build NetBSD 8.1 i386 packages on amd64 using an chroot 
with libkver. Most things seem to compile but unfortunately the rust 
bootstrap compiler fails with:

running: /pkg_comp/obj/pkgsrc/lang/rust/default/rust-bootstrap/bin/cargo 
build --manifest-path 
/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/Cargo.toml 
--frozen
thread '<unnamed>' panicked at 'failed to allocate a guard page', 
src/libstd/sys/unix/thread.rs:327:17
note: Run with `RUST_BACKTRACE=1` environment variable to display a 
backtrace.
fatal runtime error: failed to initiate panic, error 5
Traceback (most recent call last):
  File "./x.py", line 11, in <module>
    bootstrap.main()
  File 
"/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/bootstrap.py", 
line 849, in main
    bootstrap(help_triggered)
  File 
"/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/bootstrap.py", 
line 820, in bootstrap
    build.build_bootstrap()
  File 
"/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/bootstrap.py", 
line 656, in build_bootstrap
    run(args, env=env, verbose=self.verbose)
  File 
"/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/bootstrap.py", 
line 141, in run
    raise RuntimeError(err)
RuntimeError: failed to run: 
/pkg_comp/obj/pkgsrc/lang/rust/default/rust-bootstrap/bin/cargo build 
--manifest-path 
/pkg_comp/obj/pkgsrc/lang/rust/default/rustc-1.35.0-src/src/bootstrap/Cargo.toml 
--frozen
*** Error code 1

Google searches suggest that there were rust bugs in this area but they 
all seem to be referencing much older versions and indicate that the bug 
is fixed. Does amd64 have a different allocations requirement for the 
stack guard?

Any suggestions how I can do this? Building rust derived packages on the 
i386 machine is a non-starter as it will take far too long (several days 
at a guess based on the fact that the AMD64 system has 16GB of ram and a 
4GHz 8 core processor. The i386 has a single core 2Ghz processor).

Mike