Revert "Merge pull request sigp#3546 from divagant-martian/cross-update"

This reverts commit ab8dad6, reversing
changes made to d1a9635.

Author: divagant-martian

Cross.toml

@@ -1,5 +1,5 @@
 [target.x86_64-unknown-linux-gnu]
-dockerfile = './scripts/cross/x86_64-unknown-linux-gnu.dockerfile'
+pre-build = ["apt-get install -y cmake clang-3.9"]
 
 [target.aarch64-unknown-linux-gnu]
-dockerfile = './scripts/cross/aarch64-unknown-linux-gnu.dockerfile'
+pre-build = ["apt-get install -y cmake clang-3.9"]