Disable conversions between portable_simd and stdarch on big-endian ARM

stdarch no longer provide SIMD on big-endian ARM due to
rust-lang/stdarch#1484

Author: Amanieu

library/portable-simd/crates/core_simd/src/vendor/arm.rs

@@ -7,10 +7,13 @@ use core::arch::arm::*;
 #[cfg(target_arch = "aarch64")]
 use core::arch::aarch64::*;
 
-#[cfg(any(
-    target_arch = "aarch64",
-    all(target_arch = "arm", target_feature = "v7"),
-))]
+#[cfg(
+    all(any(
+        target_arch = "aarch64",
+        all(target_arch = "arm", target_feature = "v7"),
+    )),
+    target_endian = "little"
+)]
 mod neon {
     use super::*;