Fix OPENSSL_NO_ASM build and promote to a cmake build flag.

Just changing preprocessor definitions doesn't exclude the assembly files.

BUG=484327

Change-Id: I98453d291a6afb8dc8beb179f931c74301a7f434
Reviewed-on: https://boringssl-review.googlesource.com/4610
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f55765c..75a75f5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,6 +110,11 @@
   set(ARCH "x86_64")
 endif()
 
+if (OPENSSL_NO_ASM)
+  add_definitions(-DOPENSSL_NO_ASM)
+  set(ARCH "generic")
+endif()
+
 add_subdirectory(crypto)
 add_subdirectory(ssl)
 add_subdirectory(ssl/test)