Rollup merge of #49125 - NovemberZulu:master, r=alexcrichton

rustbuild: Ship libsynchronization

Hot on the heels of #49044 comes similar issue with libsynchronization. Discovered while building clippy:
```
<skipped>
Compiling serde_derive v1.0.33
error: linking with `gcc` failed: exit code: 1
<skipped>
  = note: ld: cannot find -lsynchronization
```

r? @nikomatsakis