toolchain: remove toolchain-specific stripping

We already handle the stripping of libraries in $(TARGET_DIR) at the
global level, so there's no need to have toolchain-specific option and
code for this.

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>