fix undefined_arch error in Xcode 10 beta (#19841)

Summary:
Fixes #19774

I spotted that in  Xcode 10 beta `CURRENT_ARCH` is set to string `undefined_arch`.  This PR will add fallback based on platform name (simulators are `x86_64` and everything since iPhone 5s is `arm64`).
Closes #19841

Differential Revision: D8619897

Pulled By: hramos

fbshipit-source-id: ed2ebaca105c6dcb40099f1a4aebe34d0660130c

Author: futuun

scripts/ios-configure-glog.sh

@@ -2,7 +2,18 @@
 set -e
 
 PLATFORM_NAME="${PLATFORM_NAME:-iphoneos}"
-CURRENT_ARCH="${CURRENT_ARCH:-armv7}"
+CURRENT_ARCH="${CURRENT_ARCH}"
+
+if [ -z "$CURRENT_ARCH" ] || [ "$CURRENT_ARCH" == "undefined_arch" ]; then
+    # Xcode 10 beta sets CURRENT_ARCH to "undefined_arch", this leads to incorrect linker arg.
+    # it's better to rely on platform name as fallback because architecture differs between simulator and device
+
+    if [[ "$PLATFORM_NAME" == *"simulator"* ]]; then
+        CURRENT_ARCH="x86_64"
+    else 
+        CURRENT_ARCH="armv7"
+    fi
+fi
 
 export CC="$(xcrun -find -sdk $PLATFORM_NAME cc) -arch $CURRENT_ARCH -isysroot $(xcrun -sdk $PLATFORM_NAME --show-sdk-path)"
 export CXX="$CC"