lib/systems/inspect.nix: add isILP32 predicate

I've run into a few packages that need an extra flag on platforms
where `int` has more bits than `void*` does.  I know of three such
platforms:

* [aarch64ilp32], used on both Linux and also on the [Apple Watch]
* [x32], the x86 ILP32 ABI
* [mips64n32], used on [Longsoon] and Cavium Octeon routers.

This PR introduces a predicate so the package flags can be added in
a generic way.

[Apple Watch]: https://gist.github.com/woachk/943828f37c14563a607a26116435bf27#watch
[mips64n32]: https://en.wikipedia.org/wiki/MIPS_architecture#Calling_conventions
[Longsoon]: https://en.wikipedia.org/wiki/Loongson
[x32]: https://en.wikipedia.org/wiki/X32_ABI

lib/systems/inspect.nix

@@ -48,6 +48,7 @@ rec {
 
     is32bit        = { cpu = { bits = 32; }; };
     is64bit        = { cpu = { bits = 64; }; };
+    isILP32        = map (a: { abi = { abi = a; }; }) [ "n32" "ilp32" "x32" ];
     isBigEndian    = { cpu = { significantByte = significantBytes.bigEndian; }; };
     isLittleEndian = { cpu = { significantByte = significantBytes.littleEndian; }; };