build: more stringent gold checks

This is the only site which allowed you to accidentally accept gold for targets
which are not ELFish.  For the time being, we can only add unit tests for the
host variant, so use the `SWIFT_HOST_VARIANT_SDK` to determine the target file
format.  This will make supporting cross-compiling to non-ELFish targets
simpler.

Author: compnerd

cmake/modules/AddSwiftUnittests.cmake

@@ -47,7 +47,10 @@ function(add_swift_unittest test_dirname)
       set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
         LINK_FLAGS " -fprofile-instr-generate -fcoverage-mapping")
     endif()
-  elseif(${SWIFT_ENABLE_GOLD_LINKER})
+  endif()
+
+  if(SWIFT_ENABLE_GOLD_LINKER AND
+     "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
     set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
       LINK_FLAGS " -fuse-ld=gold")
   endif()