Add guards for the architecture and OPENSSL_NO_ASM to
the assembly-language files in crypto/curve25519/asm.
The Dart compilation of BoringSSL includes all files,
because the architecture is not known when gyp is run.

Change-Id: I66f5ae525266b63b0fe3a929012b771d545779b5
Reviewed-on: https://boringssl-review.googlesource.com/7030
Reviewed-by: Adam Langley <agl@google.com>