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>