Help with building boost for ARM

I need to build some of the boost libraries for ARM. I followed the instructions here: https://www.boost.org/doc/libs/1_58_0/more/getting_started/unix-variants.html and I get the following error when running "b2 --toolset=gcc":

aarch64-linux-gnu-g++: error: unrecognized command line option ‘-m64’

If it means anything, before the error I get:
Performing configuration checks

    - 32-bit                   : no  (cached)
    - 64-bit                   : yes (cached)
    - arm                      : no  (cached) // is this right?

And
Building the Boost C++ Libraries.
...
    - x86                      : no  (cached)
    - arm                      : no  (cached) // ?

My project-config.jam has:
using gcc : arm : aarch64-linux-gnu-g++ ; // also tried arm64, aarch64-linux-gnu-gcc

Any help appreciated.

Sent with [ProtonMail](https://protonmail.com) Secure Email.

_______________________________________________
Unsubscribe & other changes: https://lists.boost.org/mailman/listinfo.cgi/boost-build