Remove build targets from gmp, mpfr, mpc, isl, cloog, and libelf

These libraries are now going to be built in combined builds in binutils
and gcc (pass1 and final).

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>
Bump cloog to 0.18.2, and use official mirror

Signed-off-by: Bryan Hundven <bryanhundven@gmail.com>