external-toolchain: Slightly optimize toolchain extraction

Some CodeSourcery toolchains contain a huge number of locales that are
not useful, even though they account for 70-80% of the total toolchain
size. By skipping the extraction of those useless locales, we make the
toolchain extraction process slightly faster, and also make the output
directory size a lot smaller (host/opt/ is 213 MB instead of 1.5 GB
with a 2010.09 ARM CodeSourcery toolchain).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>