external-toolchain-csl: update licensing info, document exceptions
external-toolchain-csl: update licensing info, document exceptions

* Split GCC and RLE licenses and base them off different gcc versions
* Document all the details about the licenses and exceptions, so they
won't get changed by accident again

Signed-off-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>

rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal,rascalmicro/openembedded-rascal