cmd/cgo: Fix CGO related LTO bug

This patch fixes another CGO related LTO bug. This seems to come
when the programmer tries to assign a C function pointer to a local Go
variable as seen in the reproducer. It turns out we do not need to emit
_cgo_hack or our own definition of the symbol(s) within the _cgo_main.c
file as was previously done.

Fixes #43830

Author: derekparker

src/cmd/cgo/out.go

@@ -168,8 +168,10 @@ func (p *Package) writeDefs() {
 			if *gccgo {
 				fmt.Fprintf(fc, "extern byte *%s;\n", n.C)
 			} else {
-				fmt.Fprintf(fm, "extern char %s[];\n", n.C)
-				fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				if n.Kind != "fpvar" {
+					fmt.Fprintf(fm, "extern char %s[];\n", n.C)
+					fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				}
 				fmt.Fprintf(fgo2, "//go:linkname __cgo_%s %s\n", n.C, n.C)
 				fmt.Fprintf(fgo2, "//go:cgo_import_static %s\n", n.C)
 				fmt.Fprintf(fgo2, "var __cgo_%s byte\n", n.C)

src/cmd/go/testdata/script/issue_43830_2.txt

@@ -0,0 +1,28 @@
+# tests golang.org/issue/43830
+
+[!cgo] skip
+
+env CGO_CFLAGS='-flto -ffat-lto-objects'
+go build main.go
+
+-- main.go --
+
+package main
+
+import "fmt"
+
+// #include "hello.h"
+import "C"
+
+func main() {
+	hello := C.hello
+	fmt.Printf("%v\n", hello)
+}
+
+-- hello.h --
+
+#include <stdio.h>
+
+void hello(void) {
+  printf("hello\n");
+}