android: Print error when trying to build on 32-bit host.

build/android/envsetup.sh

Index: build/android/envsetup.sh
diff --git a/build/android/envsetup.sh b/build/android/envsetup.sh
index 9d1b7e6fef8857b5d47afb3dd8288031ba1e1226..cdde0ced23fba0564ca3d4838cd3fb175aa0dc64 100755
--- a/build/android/envsetup.sh
+++ b/build/android/envsetup.sh
@@ -27,14 +27,39 @@ if [[ "${ANDROID_SDK_BUILD}" -eq 1 ]]; then
   echo "Using SDK build"
 fi
 
+# Get host architecture, and abort if it is 32-bit, unless --try-32
+# is also used.
+host_arch=$(uname -m)
+case "${host_arch}" in
+  x86_64)  # pass
+    ;;
+  i?86)
+    if [[ -z "${try_32bit_host_build}" ]]; then
+      echo "ERROR: Android build requires a 64-bit host build machine."
+      echo "If you really want to try it on this machine, use the \
+--try-32bit-host flag."
+      echo "Be warned that this may fail horribly at link time, due \
+very large binaries."
+      return 1
+    else
+      echo "WARNING: 32-bit host build enabled. Here be dragons!"
+      host_arch=x86
+    fi
+    ;;
+  *)
+    echo "ERROR: Unsupported host architecture (${host_arch})."
+    echo "Try running this script on a Linux/x86_64 machine instead."
+    return 1
+esac
+
 host_os=$(uname -s | sed -e 's/Linux/linux/;s/Darwin/mac/')
 
 case "${host_os}" in
   "linux")
-    toolchain_dir="linux-x86_64"
+    toolchain_dir="linux-${host_arch}"
     ;;
   "mac")
-    toolchain_dir="darwin-x86"
+    toolchain_dir="darwin-${host_arch}"
     ;;
   *)
     echo "Host platform ${host_os} is not supported" >& 2