[embedded] Use -ffreestanding when building Unicode data tables library even on Linux

Author: kubamracek

stdlib/public/stubs/CMakeLists.txt

@@ -65,7 +65,7 @@ if(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB)
     endif()
 
     if (SWIFT_HOST_VARIANT STREQUAL "linux")
-      set(extra_c_compile_flags)
+      set(extra_c_compile_flags -ffreestanding)
     elseif (SWIFT_HOST_VARIANT STREQUAL "macosx")
       set(extra_c_compile_flags -D__MACH__ -D__APPLE__ -ffreestanding)
     endif()