linux/common-config: eBPF JIT is also available on i386

This has been the case since Linux 4.18, so all Kernels in nixpkgs
should be fine:
https://github.com/torvalds/linux/commit/03f5781be2c7b7e728d724ac70ba10799cc710d7

Full platform support list:
https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/sysctl/net.rst#bpf_jit_enable

pkgs/os-specific/linux/kernel/common-config.nix

@@ -37,7 +37,7 @@ let
   whenPlatformHasEBPFJit = lib.mkIf (
     stdenv.hostPlatform.isAarch32
     || stdenv.hostPlatform.isAarch64
-    || stdenv.hostPlatform.isx86_64
+    || stdenv.hostPlatform.isx86
     || (stdenv.hostPlatform.isPower && stdenv.hostPlatform.is64bit)
     || (stdenv.hostPlatform.isMips && stdenv.hostPlatform.is64bit)
   );