[lld/mac] Use xxhash instead of MD5 for computing the UUID

15% faster for linking Chromium's base_unittests.txt, according to ministat:

```
    N           Min           Max        Median           Avg        Stddev
x  10      0.650213    0.69287586    0.65793395    0.66127126   0.012365407
+  10    0.54993701    0.59006906    0.55885506    0.56146643   0.013215349
Difference at 95.0% confidence
        -0.0998048 +/- 0.0120244
        -15.0929% +/- 1.81838%
        (Student's t, pooled s = 0.0127974)
```

And matches what we do on the other ports.

Differential Revision: https://reviews.llvm.org/D92736