Configure: recognize arm64 machine name as a synonym for aarch64.

In particular, this sets a reasonable cacheline size on FreeBSD and macOS,
which prefer to use this name and both lack _SC_LEVEL1_DCACHE_LINESIZE.

auto/os/conf

diff --git a/auto/os/conf b/auto/os/conf
index 7c6cb69..d7f6e03 100644
--- a/auto/os/conf
+++ b/auto/os/conf
@@ -110,7 +110,7 @@
         NGX_MACH_CACHE_LINE=64
     ;;
 
-    aarch64 )
+    aarch64 | arm64)
         have=NGX_ALIGNMENT value=16 . auto/define
         NGX_MACH_CACHE_LINE=64
     ;;