Rollup merge of rust-lang#63965 - loganwendholt:linker-script-fix, r=michaelwoerister

Prevent syntax error in LD linker version script

As discussed in rust-lang#63925, there is an edge case in which an invalid LD version script is generated when building a `cdylib` with no exported symbols. This PR makes a slight modification to the  LD version script generation by first checking to see if any symbols need to be exported. If not, the `global` section of the linker script is simply omitted, and the syntax error is averted.

Author: Centril

src/librustc_codegen_ssa/back/linker.rs

@@ -430,10 +430,13 @@ impl<'a> Linker for GccLinker<'a> {
             // Write an LD version script
             let res: io::Result<()> = try {
                 let mut f = BufWriter::new(File::create(&path)?);
-                writeln!(f, "{{\n  global:")?;
-                for sym in self.info.exports[&crate_type].iter() {
-                    debug!("    {};", sym);
-                    writeln!(f, "    {};", sym)?;
+                writeln!(f, "{{")?;
+                if !self.info.exports[&crate_type].is_empty() {
+                    writeln!(f, "  global:")?;
+                    for sym in self.info.exports[&crate_type].iter() {
+                        debug!("    {};", sym);
+                        writeln!(f, "    {};", sym)?;
+                    }
                 }
                 writeln!(f, "\n  local:\n    *;\n}};")?;
             };