Makefile: delete debug libs when debug is not enabled
Some toolchains, like the Linaro gcc7 toolchains, now install libstdc++ debug
library symbols to /lib/debug, which can be as large as the library itself.
This commit removes the extra debug content if debugging is not enabled.
Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>