illumos does not have versions

The 5.11 in uname is not something that is ever updated. There is no
versioning of the illumos system in general. Illumos prefers to rely
on feature detection.

I can't say what Solaris does these days as I do not work at Oracle;
so I left it alone.

lib/std/target.zig

@@ -271,6 +271,7 @@ pub const Target = struct {
                     .glsl450, // TODO: GLSL versions
                     .vulkan,
                     .plan9,
+                    .illumos,
                     .other,
                     => return .{ .none = {} },
 
@@ -331,7 +332,7 @@ pub const Target = struct {
                             .max = .{ .major = 6, .minor = 4, .patch = 0 },
                         },
                     },
-                    .solaris, .illumos => return .{
+                    .solaris => return .{
                         .semver = .{
                             .min = .{ .major = 5, .minor = 11, .patch = 0 },
                             .max = .{ .major = 5, .minor = 11, .patch = 0 },
@@ -381,7 +382,6 @@ pub const Target = struct {
                 .openbsd,
                 .dragonfly,
                 .solaris,
-                .illumos,
                 => return TaggedVersionRange{ .semver = self.version_range.semver },
 
                 else => return .none,