Build native library with BUCK for .NET (Mono, Xamarin)

Examples:

- macOS on macOS (Mono, Xamarin.Mac, Unity)
  - `buck build //csharp:yoganet#shared,default` -> `libyoga.dylib`
- Android (Xamarin.Android, Unity)
  - `buck build //csharp:yoganet#shared,android-armv7` -> `libyoga.so`
- iOS (Xamarin.iOS, Unity)
  - `buck build //:yoga#static,iphoneos-arm64` -> `libyoga.a`
  - `buck build //csharp:yoganet#static,iphoneos-arm64` -> `libyoganet.a`

YOGA_DEFS

@@ -17,6 +17,14 @@ CXX_LIBRARY_WHITELIST = [
   '//java:jni',
 ]
 
+BASE_COMPILER_FLAGS = [
+  '-fno-omit-frame-pointer',
+  '-fexceptions',
+  '-Wall',
+  '-Werror',
+  '-O3',
+]
+
 def yoga_dep(dep):
   return '//' + dep