vere: change lmdb mapsize for aarch64

Adds aarch64 lines to pkg/urbit/configure and defines a new macro, U3_CPU_aarch64.

That macro is used in pkg/urbit/vere/disk.c to change the lmdb mapsize from 1TB to 60GB for aarch64 on linux.

Uses #if defined instead of  #ifdef in anticipation of aarch64 on macos being a thing before the end of the year.

pkg/urbit/configure

@@ -46,6 +46,10 @@ case $(tr A-Z a-z <<< $cpu) in
   x86_64)
      defmacro U3_OS_ENDIAN_little 1
      ;;
+  aarch64)
+     defmacro U3_OS_ENDIAN_little 1
+     defmacro U3_CPU_aarch64 1
+     ;;
   *)
      echo "Unknown or unsupported CPU: '$cpu'" >&2
      exit 1

pkg/urbit/vere/disk.c

@@ -752,7 +752,11 @@ u3_disk_init(c3_c* pax_c, u3_disk_cb cb_u)
     //  "[..] on 64-bit there is no penalty for making this huge (say 1TB)."
     //
     {
-      const size_t siz_i = 1099511627776;
+      #if defined(U3_CPU_aarch64) && defined(U3_OS_linux)
+        const size_t siz_i = 64424509440;
+      #else
+        const size_t siz_i = 1099511627776;
+      #endif
 
       if ( 0 == (log_u->mdb_u = u3_lmdb_init(log_c, siz_i)) ) {
         fprintf(stderr, "disk: failed to initialize database");