Fix Plume arm builds (#427)

llvm latest svn commit is broken as of 2019/01/12. This patch pins the llvm+lld versions used for arm compiles.

It also tweaks the architecture detection to be more reliable in multi-lib/multi-arch situations where the CPU is 64bit but the runtime is 32bit.

script/plume-front.sh

@@ -1,8 +1,24 @@
 #!/bin/bash
 
-ARCH=`arch`
+ARCH=$(python <<EOF
+from __future__ import print_function
+import platform
+processor = platform.machine()
+architecture = platform.architecture()
+if processor == 'aarch64':
+    # Mutli arch arm support is why this 32bit check is present
+    if '32bit' in architecture:
+        print('armv71', end='')
+    else:
+        print('aarch64', end='')
+elif processor == 'x86 64' or processor == 'x86_64':
+    print('amd64', end='')
+elif processor == 'armv7l':
+    print('armhf', end='')
+EOF
+)
 
-if [ "$ARCH" == "aarch64" -o "$ARCH" == "armv7l" ] ; then
+if [ $ARCH == "aarch64" -o $ARCH == "armv71" ] ; then
     export PATH=/opt/local/llvm/bin:${PATH}
     cd /app
     RUSTFLAGS="-C linker=lld" cargo web deploy -p plume-front