update_cpu_features: map e6500 powerpc model to LLVM's e5500

This is the closest sensible thing for scheduling purposes.

lib/std/Target/powerpc.zig

@@ -694,7 +694,7 @@ pub const cpu = struct {
     };
     pub const e6500: CpuModel = .{
         .name = "e6500",
-        .llvm_name = null,
+        .llvm_name = "e5500",
         .features = featureSet(&[_]Feature{
             .@"64bit",
             .altivec,

tools/update_cpu_features.zig

@@ -3165,7 +3165,7 @@ const targets = [_]ArchTarget{
                 },
             },
             .{
-                .llvm_name = null,
+                .llvm_name = "e5500",
                 .zig_name = "e6500",
                 .features = &.{
                     "64bit",