Fix selected issues with non-4.9 gcc builds

Fix some issues that crop up when using build-gcc.sh on post-4.9
versions of GCC (for triage/bug-reproduction purposes): trunk gcc
calls isl directly, no longer through "cloog", and requires updated
version of ISL. Resulting build still fails compiling libatomic,
however the cross compiler itself is still usable.

Change-Id: Ieff549580777a01891ed62b59ff97db610eae42b