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

Summary:
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`
Closes https://github.com/facebook/yoga/pull/282

Reviewed By: emilsjolander

Differential Revision: D4333811

Pulled By: splhack

fbshipit-source-id: 54f4364d93b2419d7504392eb558296a5d06772f

BUCK

@@ -7,14 +7,6 @@
 
 include_defs('//YOGA_DEFS')
 
-BASE_COMPILER_FLAGS = [
-  '-fno-omit-frame-pointer',
-  '-fexceptions',
-  '-Wall',
-  '-Werror',
-  '-O3',
-]
-
 GMOCK_OVERRIDE_FLAGS = [
   # gmock does not mark mocked methods as override, ignore the warnings in tests
   '-Wno-inconsistent-missing-override',