Support x86_64-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

@@ -2037,6 +2037,8 @@ impl Build {
                         }
 
                         cmd.push_cc_arg(format!("--target={}", target).into());
+                    } else if target == "x86_64-unknown-linux-gnu" {
+                        cmd.push_cc_arg("--target=x86_64-linux-gnu".into());
                     } else {
                         cmd.push_cc_arg(format!("--target={}", target).into());
                     }