external-toolchain: handle libstdc++/libgcc_s for BR toolchains

Most toolchains have their libraries either in /lib or /usr/lib
relative to their ARCH_SYSROOT_DIR. Buildroot toolchains, however,
have basic libraries in /lib, and libstdc++/libgcc_s in
/usr/<target-name>/lib(64).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>