cmd/cgo: remove unnecessary space in cgo export header

The cgo header has an unnecessary space in the exported function
definition on non-windows goos.

This was introduced in go1.16 so it would be good to fix it before
release.

Example:

// Current behavior, notice there is an unecessary space
// between extern and void
extern  void Foo();

// With this CL
extern void Foo();

Change-Id: Ic2c21f8d806fe35a7be7183dbfe35ac605b6e4f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/283892
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Katie Hockman <[email protected]>

src/cmd/cgo/out.go

@@ -953,9 +953,9 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		// Build the wrapper function compiled by gcc.
 		gccExport := ""
 		if goos == "windows" {
-			gccExport = "__declspec(dllexport)"
+			gccExport = "__declspec(dllexport) "
 		}
-		s := fmt.Sprintf("%s %s %s(", gccExport, gccResult, exp.ExpName)
+		s := fmt.Sprintf("%s%s %s(", gccExport, gccResult, exp.ExpName)
 		if fn.Recv != nil {
 			s += p.cgoType(fn.Recv.List[0].Type).C.String()
 			s += " recv"