crypto: curve25519 - x86_64 library and KPP implementations

This implementation is the fastest available x86_64 implementation, and
unlike Sandy2x, it doesn't requie use of the floating point registers at
all. Instead it makes use of BMI2 and ADX, available on recent
microarchitectures. The implementation was written by Armando
Faz-Hernández with contributions (upstream) from Samuel Neves and me,
in addition to further changes in the kernel implementation from us.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Samuel Neves <sneves@dei.uc.pt>
Co-developed-by: Samuel Neves <sneves@dei.uc.pt>
[ardb: - move to arch/x86/crypto
       - wire into lib/crypto framework
       - implement crypto API KPP hooks ]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/Kconfig b/crypto/Kconfig
index fab259d..472c2ad 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -269,6 +269,12 @@
 	select CRYPTO_KPP
 	select CRYPTO_LIB_CURVE25519_GENERIC
 
+config CRYPTO_CURVE25519_X86
+	tristate "x86_64 accelerated Curve25519 scalar multiplication library"
+	depends on X86 && 64BIT
+	select CRYPTO_LIB_CURVE25519_GENERIC
+	select CRYPTO_ARCH_HAVE_LIB_CURVE25519
+
 comment "Authenticated Encryption with Associated Data"
 
 config CRYPTO_CCM