Re-enable system linker hack

It is now possible to force linking with system linker `ld` instead
of the LLVM `lld` linker when building natively on the target. This
can be done at each stage by specifying `--system-linker-hack` flag,
and can be useful on platforms where `lld` fails to operate properly
such as macOS 11 Big Sur on ARM64 where every binary/dylib is expected
to be codesigned.

Some example invocations for each stage of compilation of Zig
toolchain:

```
cmake .. -DCMAKE_PREFIX_PATH=/path/to/llvm -DSYSTEM_LINKER_HACK=1
```

```
build/zig build test --system-linker-hack
```

```
build/zig build --prefix $(pwd)/stage2 -Denable-llvm
--system-linker-hack
```

```
build/zig build-exe hello.zig --system-linker-hack
```

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