std.Target: Fix Abi.default() for mips64/mips64el.

This is necessary after:

* 270fbbcd86b02fcd02ad9b818d9de39dfe671754
* 11915ae48e44cfd9d4e5c5823919fb24951e68f9

lib/std/Target.zig

@@ -862,6 +862,9 @@ pub const Abi = enum {
                 .mips,
                 .mipsel,
                 => .musleabi,
+                .mips64,
+                .mips64el,
+                => .muslabi64,
                 else => .musl,
             },
             .rtems => switch (arch) {