stdenv/linux: Link statically against isl & cloog

Otherwise this fails on ARM:

/nix/store/jipqp9739n7wrjz40igbk85pqk13s0ad-binutils-2.23.1/bin/ld: /nix/store/92pdpqrqkdf8wjciq1cisvsp8kdz8p2i-gmp-5.1.3/lib/libgmp.a(mp_get_fns.o): relocation R_ARM_MOVW_ABS_NC against `__gmp_allocate_func' can not be used when making a shared object; recompile with -fPIC
/nix/store/92pdpqrqkdf8wjciq1cisvsp8kdz8p2i-gmp-5.1.3/lib/libgmp.a: could not read symbols: Bad value
collect2: error: ld returned 1 exit status
make[2]: *** [libisl.la] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory `/tmp/nix-build-isl-0.11.1.drv-3/isl-0.11.1'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/tmp/nix-build-isl-0.11.1.drv-3/isl-0.11.1'
make: *** [all] Error 2
builder for ‘/nix/store/a8ghniifd8d8agqx0cqsh41daa08v11c-isl-0.11.1.drv’ failed with exit code 2