update_cpu_features: rename generic xtensa model to xtensa

Since it's just the base Xtensa ISA.

lib/std/Target.zig

@@ -2079,6 +2079,7 @@ pub const Cpu = struct {
                 .x86 => &x86.cpu.i386,
                 .x86_64 => &x86.cpu.x86_64,
                 .xcore => &xcore.cpu.xs1b,
+                .xtensa, .xtensaeb => &xtensa.cpu.xtensa,
                 inline else => |a| &@field(Target, @tagName(a.family())).cpu.generic,
             };
         }

lib/std/Target/xtensa.zig

@@ -339,8 +339,8 @@ pub const cpu = struct {
             .timers1,
         }),
     };
-    pub const generic: CpuModel = .{
-        .name = "generic",
+    pub const xtensa: CpuModel = .{
+        .name = "xtensa",
         .llvm_name = "generic",
         .features = featureSet(&[_]Feature{}),
     };

tools/update_cpu_features.zig

@@ -4939,6 +4939,12 @@ const targets = [_]ArchTarget{
                 .omit = true,
             },
         },
+        .model_overrides = &.{
+            .{
+                .llvm_name = "generic",
+                .zig_name = "xtensa",
+            },
+        },
     },
 };