fix building on amd64 by Sun Studio 11 and more early versions

auto/cc/sunc

diff --git a/auto/cc/sunc b/auto/cc/sunc
index f7eb880..d464c6e 100644
--- a/auto/cc/sunc
+++ b/auto/cc/sunc
@@ -39,7 +39,7 @@
 else
     ngx_sparc32="-xarch=v8plus"
     ngx_sparc64="-xarch=v9"
-    ngx_amd64="-amd64"
+    ngx_amd64="-xarch=amd64"
 fi
 
 case "$NGX_MACHINE" in