linux: config.in: add comment for Arm Cortex-M

When binutils > 2.28 are selected on Arm Cortex-M cpu,
linux kernel does not boot due to a new implementation
of 'adr pseudo instruction' on binutils.

Bugzilla thread: https://bugs.busybox.net/show_bug.cgi?id=11051



Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>