How to build a toolchain for arm cortex-m and cortex-r

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Crossdev can almost build a functioning toolchain for embedded arm development. The toolchain can be created with the following steps:

Step 1:

root #crossdev --lenv 'USE="nano -nls -threads -unicode"' -s3 -t arm-unknown-eabi

Step 2:

root #crossdev --lenv 'USE="nano -nls -threads -unicode"' --genv 'USE="cxx -nls -nptl -pch -pie -ssp" EXTRA_ECONF="--with-multilib-list=rmprofile --disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath --disable-shared --disable-threads --disable-tls"' -s4 -t arm-unknown-eabi

Step 3:

root #emerge --ask cross-arm-unknown-eabi/newlib

Step 4:

root #crossdev --lenv 'USE="nano -nls -threads -unicode"' --genv 'USE="cxx -nls -nptl -pch -pie -ssp" EXTRA_ECONF="--with-multilib-list=rmprofile --disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath --disable-shared --disable-threads --disable-tls"' -s4 --ex-gdb -t arm-unknown-eabi

Now you'll have a functioning multilib / multiarch for embedded arm development with small code output size.

Big thanks to our forum user rapsure who posted this instructions here.