Fix building rustc and the rustc test suite

Author: bjorn3

scripts/setup_rust_fork.sh

@@ -41,3 +41,6 @@ popd
 # Allow the testsuite to use llvm tools
 host_triple=$(rustc -vV | grep host | cut -d: -f2 | tr -d " ")
 export LLVM_BIN_DIR="$(rustc --print sysroot)/lib/rustlib/$host_triple/bin"
+
+# local-rebuild should probably set this unconditionally
+export BOOTSTRAP_SKIP_TARGET_SANITY=1