% uname -a
Linux raspberrypi 4.1.7-v7+ #817 SMP PREEMPT Sat Sep 19 15:32:00 BST 2015 armv7l GNU/Linux
% gcc --version | head -n 1
gcc (Raspbian 4.8.4-1) 4.8.4

sum.go

package main

import "C"

//export Sum
func Sum(a, b int) int {
        return a + b
}

func main() {}

#include <stdio.h>
#include "sum.h"

int main() {
  printf("1 + 2 = %d\n", Sum(1, 2));
  return 0;
}

% go build -buildmode=c-shared -o ./sum.so ./sum.go
% gcc -c ./main.c
% gcc ./main.o ./sum.so

% ./a.out
zsh: segmentation fault  ./a.out

% go build -buildmode=c-archive -o ./sum.a ./sum.go
% gcc ./main.o ./sum.a
./sum.a(000001.o): In function `x_cgo_sys_thread_create':
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_libinit.c:20: undefined reference to `pthread_create'
./sum.a(000001.o): In function `_cgo_sys_thread_start':
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_linux_arm.c:25: undefined reference to `pthread_sigmask'
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_linux_arm.c:33: undefined reference to `pthread_attr_getstacksize'
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_linux_arm.c:36: undefined reference to `pthread_create'
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_linux_arm.c:38: undefined reference to `pthread_sigmask'
./sum.a(000001.o): In function `x_cgo_init':
/home/pi/tmp/go1.5/src/runtime/cgo/gcc_linux_arm.c:66: undefined reference to `pthread_attr_getstacksize'
collect2: error: ld returned 1 exit status

% gcc -pthread ./main.o ./sum.a
% ./a.out
1 + 2 = 3

% gcc -pthread ./main.o ./sum.so
% ./a.out
zsh: segmentation fault  ./a.out