Fix for MacOS AArch64 builds

Calls to getauxval were guarded just by `__aarch64__`, but we only
actually include the appropriate header if SPEAD2_USE_SVE_STREAM is
defined (and it is never defined on MacOS because I don't know how to
detect the feature on MacOS). So builds that explicitly disabled SVE
would also have been broken.

src/common_memcpy.cpp

@@ -138,7 +138,7 @@ void *memcpy_nontemporal_sve(void * __restrict__ dest, const void * __restrict__
 #endif // SPEAD2_USE_SVE_STREAM
 
 extern "C" void *(*spead2_resolve_memcpy_nontemporal(
-#ifdef __aarch64__
+#if SPEAD2_USE_SVE_STREAM
     std::uint64_t hwcaps  // See System V AVI for AArch64
 #endif
 ))(void *, const void *, std::size_t) noexcept
@@ -188,7 +188,7 @@ void *memcpy_nontemporal(void * __restrict__ dest, const void * __restrict__ src
 
 void *memcpy_nontemporal(void * __restrict__ dest, const void * __restrict__ src, std::size_t n) noexcept
 {
-#ifdef __aarch64__
+#if SPEAD2_USE_SVE_STREAM
     static void *(*memcpy_nontemporal_ptr)(void * __restrict__ dest, const void * __restrict__ src, std::size_t n) noexcept =
         spead2_resolve_memcpy_nontemporal(getauxval(AT_HWCAPS));
 #else