cross-compiling 1.58 for arm64

I am cross-compiling 1.58 for 64-bit ARM. I have done this before (with some help - http://boost.2283326.n4.nabble.com/Help-with-building-boost-for-ARM-td4703570.html). This time, I am unable to follow my own notes or the docs on cross-compilation.

I add user-config.jam as described here: https://boostorg.github.io/build/manual/develop/index.html#bbv2.tasks.crosscompile
with:
using gcc : arm : aarch64-linux-gnu-g++ ;

And compile with:
./bootstrap.sh
./b2 -q --without-context --without-coroutine -s NO_BZIP2=1 toolset=gcc architecture=arm address-model=64

This produces an x86 build (it used to produce an arm build on a different machine, and I don't know what's changed).

If I try toolset=gcc-arm, I get:
.../boost_1_58_0/tools/build/src/tools/gcc.jam:121: in gcc.init from module gcc
error: toolset gcc initialization:
error: version 'arm' requested, but 'g++-arm' not found and version '5.4.0' of default 'g++' does not match

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