gcc-runtime-external: symlink TARGET_SYS, not move
There are two cases where TARGET_SYS might not match
EXTERNAL_TARGET_SYS. In one case, the external toolchain simply used
a different one than we did, and we need to align expectations, but in
another, they may not match due to multilib usage, and in such a case we
likely need both available.
Signed-off-by: Christopher Larson <chris_larson@mentor.com>