How to test aarch64 when building a cross-compiler?

[Andrew Dean via gcc]

Based on https://www.gnu.org/software/hurd/hurd/glibc.html, I'm using glibc/scripts/build-many-glibcs.py targeting aarch64-linux-gnu as so:

build-many-glibcs.py build_dir checkout --keep all

build-many-glibcs.py build_dir host-libraries --keep all -j 12

build-many-glibcs.py build_dir compilers aarch64-linux-gnu --keep all -j 12 --full-gcc
build-many-glibcs.py build_dir glibcs aarch64-linux-gnu --keep all -j 12

This completes successfully. However, when I then try to run the gcc tests like so:
runtest --outdir . --tool gcc --srcdir /path/to/gcc/gcc/testsuite aarch64.exp --target aarch64-linux-gnu --target_board aarch64-sim --tool_exec /path_to/build_dir/install/compilers/aarch64-linux-gnu/bin/aarch64-glibc-linux-gnu-gcc --verbose -v

I get errors like this:

aarch64-glibc-linux-gnu-gcc: fatal error: cannot read spec file 'rdimon.specs': No such file or directory

I can see that the rdimon.specs flag is added based on this line in aarch64-sim.exp:

set_board_info ldflags 	"[libgloss_link_flags] [newlib_link_flags] -specs=rdimon.specs"

I've tried searching for how to address this, but so far unsuccessfully. Does anybody know what I'm missing here?

Thanks,
Andrew