Do not include GCC source code in source tarballs

The licensing story is unclear, it makes the archive much larger, and we should not need it for building anything in the tarballs (yet).

(cherry picked from commit f854f34)

Author: Kobzol

src/bootstrap/src/core/build_steps/dist.rs

@@ -995,7 +995,18 @@ impl Step for PlainSourceTarball {
         ];
         let src_dirs = ["src", "compiler", "library", "tests"];
 
-        copy_src_dirs(builder, &builder.src, &src_dirs, &[], plain_dst_src);
+        copy_src_dirs(
+            builder,
+            &builder.src,
+            &src_dirs,
+            &[
+                // We don't currently use the GCC source code for building any official components,
+                // it is very big, and has unclear licensing implications due to being GPL licensed.
+                // We thus exclude it from the source tarball from now.
+                "src/gcc",
+            ],
+            plain_dst_src,
+        );
 
         // Copy the files normally
         for item in &src_files {