configure: add basic support for ARM AArch64

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -1064,6 +1064,7 @@ THREADS_LIST='
 '
 
 ARCH_LIST='
+    aarch64
     alpha
     arm
     avr32
@@ -2460,6 +2461,9 @@ fi
 
 # Deal with common $arch aliases
 case "$arch" in
+    aarch64|arm64)
+        arch="aarch64"
+    ;;
     arm*)
         arch="arm"
     ;;
@@ -2635,6 +2639,17 @@ elif enabled avr32; then
         ;;
     esac
 
+elif enabled aarch64; then
+
+    case $cpu in
+        armv*)
+            cpuflags="-march=$cpu"
+        ;;
+        *)
+            cpuflags="-mcpu=$cpu"
+        ;;
+    esac
+
 fi
 
 add_cflags $cpuflags