excellent @johannes, I'll see if I can give it a test, and if I have issues I'll looking into either reporting or fixing 
In particular I'll take a look at the module stuff, and look into resolving the MI stuff.
installation
ok, Im not quite sure what @johannes's plan is on how users are going to install gcc-arm-none-eabi
(or if we are going to recommend a version, since using the latest, might cause some oddities between users!)
once confirmed, I'll looking into updating the installation instructions where necessary.
EDIT : see post below this one- for simple install
if you use gcc-arm-none-eabi-7-2018-q2-update, and place in /Applications, then it will work 'out of the box'
following is useful if you want to use a different version of gcc-arm, or change where it is installed.
for now... here is what I did (macOS)
a) download gcc-arm-none-eabi
from https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads
current is, gcc-arm-none-eabi-9-2019-q4-major, which I'll use for this example.
b) untar/zip it...
this can be done with apples archive utility, so pretty much should be just double click on it
creates a folder called something like gcc-arm-none-eabi-9-2019-q4-major
c) move to a suitable location
I personally would recommend (and use in this example) /usr/local
so we end up with
/usr/local/gcc-arm-none-eabi-9-2019-q4-major
d) add bin dir to path in axoloti preferences
additional path : /usr/local/gcc-arm-none-eabi-9-2019-q4-major
(following is uninteresting, unless you are planning on helping with axoloti development )
dev installation
(again macOS, possibly similar on windows?)
as before we use build.sh to build axoloti, but some additional notes
a) newer Chibios
if you have previously built axoloti, you will need to delete this to get it to redownload,
though frankly, assuming you dont have local changes, its easier to reclone the axoloti repo 
b) arm-none-eabi-gcc
as for running axoloti, we now need to supply arm-none-eabi-gcc,
but command line doesn't use preferences,instead platform_osx/paths.sh contains the path expect to find arm-none-eabi-gcc,
it defaults to /Applications/gcc-arm-none-eabi-7-2018-q2-update/bin
ok, Id never put the compiler in /Applications, and ive a newer version , so I changed it to
#export PATH=/Applications/gcc-arm-none-eabi-7-2018-q2-update/bin:${axoloti_runtime}/platform_osx/bin:$PATH
export PATH=/usr/local/gcc-arm-none-eabi-9-2019-q4-major/bin:${axoloti_runtime}/platform_osx/bin:$PATH
@johannes we probably need to change this, otherwise, you'll get updates/PR with this line constantly changing as people reflect their own environment.
probably we should decide on a location (/usr/local seems natural on macOS these days),
then either
a specific version to support
or (and i think better) a symbolic link
e.g.
/usr/local/bin/axoloti-gcc-arm-none-eabi -> /usr/local/gcc-arm-none-eabi-9-2019-q4-major
this way, devs can keep diff versions of gcc-arm-none-eabi, and select one to use for axoloti (dev)
(this is similar to the way linux has a way to select default gcc etc)
(i'll raise an git issue, so we can device what to do )
