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>

Some build systems don't like two targets with the same base name and
the curve25519 code had x25519-x86_64.[Sc].

Change-Id: If8382eb84996d7e75b34b28def57829d93019cff
Reviewed-on: https://boringssl-review.googlesource.com/6878
Reviewed-by: Adam Langley <agl@google.com>

Nexus 7 goes from 1002.8 ops/sec to 4704.8 at a cost of 10KB of code.
(It'll actually save code if built with -mfpu=neon because then the
generic version can be discarded by the compiler.)

Change-Id: Ia6d02efb2c2d1bb02a07eb56ec4ca3b0dba99382
Reviewed-on: https://boringssl-review.googlesource.com/6524
Reviewed-by: Adam Langley <agl@google.com>