Support Alpine linux

Requires compiling for musl, not glibc.

**Workaround**
Users can remove libuv from their app and rely on the system's installed version of libuv instead.
```xml
<ItemGroup>
  <PackageReference Include="Libuv" Version="1.10.0" ExcludeAssets="Native" />
</ItemGroup>
```
Then, install libuv on host machines.
```
apk add libuv
ln -s /usr/lib/libuv.so.1 /usr/lib/libuv.so
```