Change default apple toolchain from darwin_x86_64 to ios_x86_64.

This means that in the absence of "--config=" (or the "--cpu=" and "--ios_cpu=" flags that it indicates), c++ code using //tools/osx/crosstool will default to being built for ios_x86_64 instead of darwin_x86_64.

This is necessary to ensure that once the crosstool is used for objc, the default behavior of objc_library building for ios_x86_64 will be maintained.

--
MOS_MIGRATED_REVID=132326009

compile.sh

@@ -91,7 +91,7 @@ fi
 
 if [[ $PLATFORM == "darwin" ]] && \
     xcodebuild -showsdks 2> /dev/null | grep -q '\-sdk iphonesimulator'; then
-  EXTRA_BAZEL_ARGS="${EXTRA_BAZEL_ARGS-} --define IPHONE_SDK=1"
+  EXTRA_BAZEL_ARGS="${EXTRA_BAZEL_ARGS-} --define IPHONE_SDK=1 --cpu=darwin_x86_64 --ios_cpu=x86_64"
 fi
 source scripts/bootstrap/bootstrap.sh