Set --noexecstack for assembly files in the standalone build.

See also upstream's 2966c2ec31e81187da3fbbe1499a6aa3acfd355f.

Change-Id: Iad0a0f11accb4fa2bd93667239dd7462f9fdbd7f
Reviewed-on: https://boringssl-review.googlesource.com/7180
Reviewed-by: Adam Langley <agl@google.com>

crypto/CMakeLists.txt

@@ -22,6 +22,7 @@ elseif(UNIX)
   endif()
   set(ASM_EXT S)
   enable_language(ASM)
+  set(CMAKE_ASM_FLAGS "-Wa,--noexecstack")
 else()
   if (CMAKE_CL_64)
     message("Using nasm")