Handle non-fat frameworks in iOS app framework thinning (#7950)

Support for thinning app frameworks to the target architecture was added
in 708909fc6b4a6f50d024c0d36843883284001219. This commit adds support
and error-checking for non-fat frameworks that are not of the target
architecture. In such cases, we now fail the build, and emit an error
message and the contents of lipo -info for the affected framework.

packages/flutter_tools/bin/xcode_backend.sh

diff --git a/packages/flutter_tools/bin/xcode_backend.sh b/packages/flutter_tools/bin/xcode_backend.sh
index d4d7b3f..e4d4fe5 100755
--- a/packages/flutter_tools/bin/xcode_backend.sh
+++ b/packages/flutter_tools/bin/xcode_backend.sh
@@ -141,13 +141,22 @@
   local all_executables=()
   for arch in $archs; do
     local output="${executable}_${arch}"
-    lipo -output "${output}" -extract "${arch}" "${executable}"
-    if [[ $? == 0 ]]; then
-      all_executables+=("${output}")
+    local lipo_info=$(lipo -info "${executable}")
+    if [[ "${lipo_info}" == "Non-fat file:"* ]]; then
+      if [[ "${lipo_info}" != *"${arch}" ]]; then
+        echo "Non-fat binary ${executable} is not ${arch}. Running lipo -info:"
+        echo "${lipo_info}"
+        exit 1
+      fi
     else
-      echo "Failed to extract ${arch} for ${executable}. Running lipo -info:"
-      lipo -info "${executable}"
-      exit 1
+      lipo -output "${output}" -extract "${arch}" "${executable}"
+      if [[ $? == 0 ]]; then
+        all_executables+=("${output}")
+      else
+        echo "Failed to extract ${arch} for ${executable}. Running lipo -info:"
+        lipo -info "${executable}"
+        exit 1
+      fi
     fi
   done