wireguard: bump to 0.0.20180910

wireguard: bump to 0.0.20180910

* curve25519: arm: do not modify sp directly
* compat: support neon.h on old kernels
* compat: arch-namespace certain includes
* compat: move simd.h from crypto to compat since it's going upstream

This fixes a decent amount of compat breakage and thumb2-mode breakage
introduced by our move to Zinc.

* crypto: use CRYPTOGAMS license

Rather than using code from OpenSSL, use code directly from AndyP.

* poly1305: rewrite self tests from scratch
* poly1305: switch to donna

This makes our C Poly1305 implementation a bit more intensely tested and also
faster, especially on 64-bit systems. It also sets the stage for moving to a
HACL* implementation when that's ready.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
(backported from a54f492d0cf1f9bff1dd449961441e789c724995)