Fix nightly build (dalek-cryptography#619)

* Fix nightly build

* Add nightly feature constraint so AVX-512 requires either x86 or x86_64

Co-authored-by: Tony Arcieri <bascule@gmail.com>

* fmt

---------

Co-authored-by: Michael Rosenberg <micro@fastmail.com>
Co-authored-by: Tony Arcieri <bascule@gmail.com>
Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>

curve25519-dalek/src/lib.rs

@@ -10,7 +10,14 @@
 // - Henry de Valence <hdevalence@hdevalence.ca>
 
 #![no_std]
-#![cfg_attr(all(curve25519_dalek_backend = "simd", nightly), feature(stdsimd))]
+#![cfg_attr(
+    all(
+        curve25519_dalek_backend = "simd",
+        nightly,
+        any(target_arch = "x86", target_arch = "x86_64")
+    ),
+    feature(stdarch_x86_avx512)
+)]
 #![cfg_attr(
     all(curve25519_dalek_backend = "simd", nightly),
     feature(avx512_target_feature)