ARM + Linux territory. ...
:
get some hands on experience with low-level systems programming.
Arguably, once you have linux, you no longer do "low-level systems programming"; you mostly do "linux systems administration." (And I've been amused to watch beginning arduino users who asked basic programming questions move to Raspberry Pi and start asking basic linux admin questions.)
I guess there are things that are comparatively low-level: writing network daemons or middleware that speaks to sockets and /dev/etc instead of being off in the GUI stratosphere, but it's a lot different than programming an arduino. (Given an I2C device driver provided to you, you'd still have some effort required to be able to interface a 9DoF sensor array to your high-level SW.)
There COULD be a community of people writing device-driver level firmware for the small linux boards, which is more what I'd consider to be really low level. But I'm not aware of anything.