toolchain-external: also put libgcc_s.so unconditionally in TOOLCHAIN_EXTERNAL_LIBS

libgcc_s.so is now added to TOOLCHAIN_EXTERNAL_LIBS for glibc/uclibc
in one place, and for musl in another place. Bottom line: it should be
in TOOLCHAIN_EXTERNAL_LIBS unconditionally.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>