I don't recommend it, but you can preprocess Go code.

preprocess.go

/*
You can preprocess this using the following: 
  gcc -E -x c -P -C main.go
*/

/* abs.h */
#define ABS(N, T) func Abs##N(v T) T { \
;       if v < 0 { \
;               return -v \
;       } \
;       return v \
}

/* main.go */
package main

import (
        "fmt"
)

#include "abs.h"
ABS(Int, int)
ABS(Float, float)

func main() {
        fmt.Println(AbsInt(-42), AbsFloat(-3.1415))
}