stubs: configure flags based on SDK target

Due to the custom build system implemented in CMake for Swift, we cannot
properly detect the target and set flags appropriately.  Instead, assume
that if the primary variant is an Apple target, that all targets are
Apple variants.  This fixes cross-compilation on macOS.

Author: compnerd

stdlib/public/stubs/CMakeLists.txt

@@ -41,7 +41,7 @@ add_swift_target_library(swiftStdlibStubs
                   INSTALL_IN_COMPONENT
                     stdlib)
 
-if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+if("${SWIFT_PRIMARY_VARIANT_SDK}" IN_LIST SWIFT_APPLE_PLATFORMS)
   set_property(SOURCE
                  SwiftNativeNSXXXBaseARC.m
                APPEND_STRING PROPERTY COMPILE_FLAGS