update_cpu_features: don't map generic_la32 loongarch model to LLVM's generic-la32

Clang doesn't recognize it.

lib/std/Target/loongarch.zig

@@ -131,7 +131,7 @@ pub const all_features = blk: {
 pub const cpu = struct {
     pub const generic_la32: CpuModel = .{
         .name = "generic_la32",
-        .llvm_name = "generic-la32",
+        .llvm_name = null,
         .features = featureSet(&[_]Feature{
             .@"32bit",
         }),

tools/update_cpu_features.zig

@@ -2626,6 +2626,10 @@ const targets = [_]ArchTarget{
                 .llvm_name = "generic",
                 .omit = true,
             },
+            .{
+                .llvm_name = "generic-la32",
+                .omit = true,
+            },
             .{
                 .llvm_name = "generic-la64",
                 .omit = true,
@@ -2640,6 +2644,13 @@ const targets = [_]ArchTarget{
             },
         },
         .extra_models = &.{
+            .{
+                .llvm_name = null,
+                .zig_name = "generic_la32",
+                .features = &.{
+                    "32bit",
+                },
+            },
             .{
                 .llvm_name = null,
                 .zig_name = "generic_la64",