[cmake] Demote fatal error to a warning when we don't know the Apple SDK in use

Sometimes, we could be building for a platform where we don't link
compiler-rt, so being able to figure out the right compiler-rt suffix
isn't necessary, but we shouldn't fail the build.

cmake/Modules/HandleCompilerRT.cmake

@@ -38,7 +38,7 @@ function(get_component_name name variable)
     elseif (CMAKE_OSX_SYSROOT MATCHES ".+WatchSimulator.+")
       set(component_name "${component_name}watchossim")
     else()
-      message(FATAL_ERROR "Unknown Apple SDK ${CMAKE_OSX_SYSROOT}, we don't know which compiler-rt library suffix to use.")
+      message(WARNING "Unknown Apple SDK ${CMAKE_OSX_SYSROOT}, we don't know which compiler-rt library suffix to use.")
     endif()
   else()
     set(component_name "${name}")