configure: detect 64-bit generic platforms

Currently, any unrecognised platform is treated as 32-bit. This should
detect *most* 64-bit platforms, namely LP64 and LLP64 ones.
Unfortunately this will not work for ILP32 ABIs on 64-bit ISAs, but
still better than nothing.

configure

@@ -5564,6 +5564,12 @@ elif enabled x86; then
         ;;
     esac
 
+else
+
+    if test_cpp_condition inttypes.h "UINTPTR_MAX >= UINT64_MAX"; then
+        enable fast_64bit
+    fi
+
 fi
 
 if [ "$cpu" != generic ]; then