acle/hints: make sevl truly available on aarch64

addresses #557 (comment)

Author: japaric

crates/core_arch/src/acle/hints.rs

@@ -46,7 +46,10 @@ pub unsafe fn __sev() {
 /// instruction. In a multiprocessor system, it is not required to affect the
 /// other processors.
 // LLVM says "instruction requires: armv8"
-#[cfg(target_feature = "v8")]
+#[cfg(any(
+    target_feature = "v8", // 32-bit ARMv8
+    target_arch = "aarch64", // AArch64
+))]
 #[inline(always)]
 pub unsafe fn __sevl() {
     hint(HINT_SEVL);