Remove unnecessary symbols and sections from *_gnu import libs

This cuts their size by more than 20% by removing what is essentially
dead-weight:
 crates/targets/i686_gnu/lib/libwindows.a   | Bin 14009016 -> 10937090 bytes
 crates/targets/x86_64_gnu/lib/libwindows.a | Bin 13879932 -> 10500116 bytes

crates/tools/gnu/src/main.rs

@@ -91,10 +91,19 @@ EXPORTS
     }
     cmd.output().unwrap();
 
-    // Work around lack of determinism in dlltool output.
+    // Work around lack of determinism in dlltool output, and at the same time remove
+    // unnecessary sections and symbols.
     std::fs::rename(output.join(format!("lib{}.a", library)), output.join("tmp.a")).unwrap();
     let mut cmd = std::process::Command::new("objcopy");
     cmd.current_dir(&output);
+    cmd.arg("--remove-section=.bss");
+    cmd.arg("--remove-section=.data");
+    cmd.arg("--strip-unneeded-symbol=.text");
+    cmd.arg("--strip-unneeded-symbol=.data");
+    cmd.arg("--strip-unneeded-symbol=.bss");
+    cmd.arg("--strip-unneeded-symbol=.idata$7");
+    cmd.arg("--strip-unneeded-symbol=.idata$5");
+    cmd.arg("--strip-unneeded-symbol=.idata$4");
     cmd.arg("tmp.a");
     cmd.arg(format!("lib{}.a", library));
     cmd.output().unwrap();