Support *-unknown-linux-gnu under hermetic_cc_toolchain

This fork provides a temporary fix for ziglang/zig#4911,
by making our current Rust builds not generate targets that `zig cc`
doesn't understand.

src/lib.rs

@@ -2206,6 +2206,15 @@ impl Build {
                         Cow::Owned(
                             target.versioned_llvm_target(&self.apple_deployment_target(target)),
                         )
+                    } else if target.vendor == "unknown" && target.os == "linux" && target.abi == "gnu" {
+                        // Special case the `<foo>-unknown-linux-gnu` target triple, since Bazel's
+                        // hermetic build system doesn't support these target quads.
+                        match target.arch {
+                            "x86_64" => Cow::Borrowed("x86_64-linux-gnu"),
+                            "aarch64" => Cow::Borrowed("aarch64-linux-gnu"),
+                            "arm64" => Cow::Borrowed("arm64-linux-gnu"),
+                            _ => panic!("Unsupported target triple - please update the cc-rs patch"),
+                        }
                     } else {
                         Cow::Borrowed(target.llvm_target)
                     };