crypto: x86/curve25519 - replace with formally verified implementation

This comes from INRIA's HACL*/Vale. It implements the same algorithm and
implementation strategy as the code it replaces, only this code has been
formally verified, sans the base point multiplication, which uses code
similar to prior, only it uses the formally verified field arithmetic
alongside reproducable ladder generation steps. This doesn't have a
pure-bmi2 version, which means haswell no longer benefits, but the
increased (doubled) code complexity is not worth it for a single
generation of chips that's already old.

Performance-wise, this is around 1% slower on older microarchitectures,
and slightly faster on newer microarchitectures, mainly 10nm ones or
backports of 10nm to 14nm. This implementation is "everest" below:

Xeon E5-2680 v4 (Broadwell)

     armfazh: 133340 cycles per call
     everest: 133436 cycles per call

Xeon Gold 5120 (Sky Lake Server)

     armfazh: 112636 cycles per call
     everest: 113906 cycles per call

Core i5-6300U (Sky Lake Client)

     armfazh: 116810 cycles per call
     everest: 117916 cycles per call

Core i7-7600U (Kaby Lake)

     armfazh: 119523 cycles per call
     everest: 119040 cycles per call

Core i7-8750H (Coffee Lake)

     armfazh: 113914 cycles per call
     everest: 113650 cycles per call

Core i9-9880H (Coffee Lake Refresh)

     armfazh: 112616 cycles per call
     everest: 114082 cycles per call

Core i3-8121U (Cannon Lake)

     armfazh: 113202 cycles per call
     everest: 111382 cycles per call

Core i7-8265U (Whiskey Lake)

     armfazh: 127307 cycles per call
     everest: 127697 cycles per call

Core i7-8550U (Kaby Lake Refresh)

     armfazh: 127522 cycles per call
     everest: 127083 cycles per call

Xeon Platinum 8275CL (Cascade Lake)

     armfazh: 114380 cycles per call
     everest: 114656 cycles per call

Achieving these kind of results with formally verified code is quite
remarkable, especialy considering that performance is favorable for
newer chips.
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>