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/Compilation.zig b/src/Compilation.zig
index d4289fad34..961f8c08d5 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -348,6 +348,8 @@ pub const InitOptions = struct {
     want_valgrind: ?bool = null,
     use_llvm: ?bool = null,
     use_lld: ?bool = null,
+    /// When set, this option will overwrite LLD with the system linker LD.
+    system_linker_hack: bool = false,
     use_clang: ?bool = null,
     rdynamic: bool = false,
     strip: bool = false,
@@ -775,6 +777,7 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
             .optimize_mode = options.optimize_mode,
             .use_lld = use_lld,
             .use_llvm = use_llvm,
+            .system_linker_hack = options.system_linker_hack,
             .link_libc = link_libc,
             .link_libcpp = options.link_libcpp,
             .objects = options.link_objects,