cmd/cgo: Define exported Go func as function

In order to resolve symbols correctly during C compilation with CGO
the compiler emits a _cgo_main.c file which holds definitions for
symbols. The problem is that the definitions in the _cgo_main.c do not
match up with what's declared in _cgo_export.c which causes compilation
to fail if using CGO_CFLAGS="-flto -ffat-lto-objects" due to an
incompatibility with link time optimization in GCC.

This patch makes it so that the symbol emitted in _cgo_main.c lines up
with the type declared in _cgo_export.c. This should be safe to do as
the actual exported C symbol ends up as an argument to crosscall2 which
defines the function signature for these exported symbols. The function
sig will be consistent in that way since CGO generates wrappers and
structs to handle argument / return value passing.

Author: derekparker

src/cmd/cgo/out.go

@@ -1028,7 +1028,7 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		fmt.Fprintf(fgo2, "//go:cgo_export_static _cgoexp%s_%s\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a *%s) {\n", cPrefix, exp.ExpName, gotype)
 
-		fmt.Fprintf(fm, "int _cgoexp%s_%s;\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fm, "void _cgoexp%s_%s(void* p){};\n", cPrefix, exp.ExpName)
 
 		if gccResult != "void" {
 			// Write results back to frame.