buildroot

toolchain-external: mass-define actual source tarball for known patterns

For some external toolchain vendors the actual source code URL can be simply
derived from the binary file URL.

Here we obtain TOOLCHAIN_EXTERNAL_ACTUAL_SOURCE_TARBALL for all Mentor and
Linaro toolchains with a few $(subst) calls.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>