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
```

1 files changed, 3 insertions, 0 deletions

diff --git a/src/link.zig b/src/link.zig
index 02433ecde7..65074924d1 100644
--- a/src/link.zig
+++ b/src/link.zig
@@ -57,6 +57,9 @@ pub const Options = struct {
     /// other objects.
     /// Otherwise (depending on `use_lld`) this link code directly outputs and updates the final binary.
     use_llvm: bool,
+    /// If this is true and `use_llvm` is true, this link code will use system linker `ld` instead of 
+    /// the LLD.
+    system_linker_hack: bool,
     link_libc: bool,
     link_libcpp: bool,
     function_sections: bool,