std.Target: Define and use lime1 as the baseline CPU model for WebAssembly.

See: https://github.com/WebAssembly/tool-conventions/pull/235

This is not *quite* using the same features as the spec'd lime1 model because
LLVM 19 doesn't have the level of feature granularity that we need for that.
This will be fixed once we upgrade to LLVM 20.

Part of #21818.

tools/update_cpu_features.zig

@@ -1033,6 +1033,20 @@ const llvm_targets = [_]LlvmTarget{
         .zig_name = "wasm",
         .llvm_name = "WebAssembly",
         .td_name = "WebAssembly.td",
+        .extra_cpus = &.{
+            .{
+                .llvm_name = null,
+                .zig_name = "lime1",
+                .features = &.{
+                    "bulk_memory",
+                    "extended_const",
+                    "multivalue",
+                    "mutable_globals",
+                    "nontrapping_fptoint",
+                    "sign_ext",
+                },
+            },
+        },
     },
     .{
         .zig_name = "x86",