Append to CMAKE_ASM_FLAGS rather than replace it.

Otherwise we clobber things like -m32.

Change-Id: I9457e4b50dc3063643c31d19c7935276b8a312a1
Reviewed-on: https://boringssl-review.googlesource.com/7209
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index b59b7a2..ad2cdd7 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -22,7 +22,7 @@
   endif()
   set(ASM_EXT S)
   enable_language(ASM)
-  set(CMAKE_ASM_FLAGS "-Wa,--noexecstack")
+  set(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} -Wa,--noexecstack")
 else()
   if (CMAKE_CL_64)
     message("Using nasm")