cmd/cgo: improve error messages

[austinderek]

Hello maintainers,

The PR aims to improve error messages in cgoCheckPointer and cgoCheckResult.

The issue details the problem in cgoCheckResult. Let me know if you want me to split the PR for cgoCheckPointer.

Here is a reproducer for cgoCheckPointer:

package main

/*
#include <stdio.h>
void print_ptr(void *p) {
    printf("Pointer: %p\n", p);
}
*/
import "C"
import "unsafe"

func main() {
    x := 10
    p := &x          // p is a Go pointer
    C.print_ptr(unsafe.Pointer(&p)) // passing Go pointer to Go pointer
}

And the error:

panic: runtime error: cgo argument has Go pointer to unpinned Go pointer

goroutine 1 [running]:
main.main.func1(...)
        /home/ariel/src/go/src/test.go:15
main.main()
        /home/ariel/src/go/src/test.go:15 +0x79
exit status 2

Fixes golang#75856
cc @ianlancetaylor, @randall77, @seankhliao

---

🔄 This is a mirror of upstream PR golang#75894