crypto: arm/curve25519 - import Bernstein and Schwabe's Curve25519 AR…

…M implementation

This comes from Dan Bernstein and Peter Schwabe's public domain NEON
code, and is included here in raw form so that subsequent commits that
fix these up for the kernel can see how it has changed. This code does
have some entirely cosmetic formatting differences, adding indentation
and so forth, so that when we actually port it for use in the kernel in
the subsequent commit, it's obvious what's changed in the process.

This code originates from SUPERCOP 20180818, available at
<https://bench.cr.yp.to/supercop.html>.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>