Allow specifying ios sdks and linker to use in engine builds. E.g,

`$flutter/flutter_osx_sdk: {"iphoneos_sdk": "14.0", "iphonesimulator_sdk": "14.0", "ld": "609"}`

sample build:
https://ci.chromium.org/raw/build/logs.chromium.org/flutter/led/weiyuhuang_google.com/71df1f9f13d3b1c6492b2b82cd77383a946cf4dad1fdde4f0f7d49385ac655c5/+/annotations?server=chromium-swarm.appspot.com
with bitcode: https://ci.chromium.org/raw/build/logs.chromium.org/flutter/led/weiyuhuang_google.com/cbd9f895997d496d122dc0dd28b72b44080e68c9abb70db03f34b3c6518bae65/+/annotations?server=chromium-swarm.appspot.com


`git cl format` is not working for me. Is there a fix for it?

Change-Id: Ic36c025aa70a55d9c1b59e4bc0d6fd3063b03388
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6960
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Jenn Magder <magder@google.com>
Commit-Queue: Weiyu Huang <weiyuhuang@google.com>