musl: fix installed symlink

'make install' creates a symlink pointing to the target file inside DESTDIR
(equivalent of `ln -s ${IMAGE}${LIBDIR}/libc.so ${IMAGE}/${LDSO_PATHNAME}`).
This obviously breaks when merged into the root filesystem (in fact paludis
is smart enough to notice this and abort).
This commit patches Makefile to strip off the DESTDIR prefix from the symlink target.