uclibc: use builtin startfiles target

Now that we've dropped 0.9.31.x we can use the startfiles /
install_startfiles targets instead of building and installing them by
hand.
Fixes internal blackfin toolchain build errors regarding crtreloc.o

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>