[release-branch.go1.15] cmd/cgo: remove unnecessary space in cgo export header
    
    The cgo header has an unnecessary space in the exported function
    definition on non-windows goos.
    
    This was introduced in go1.16 so it would be good to fix it before
    release.
    
    Example:
    
    // Current behavior, notice there is an unecessary space
    // between extern and void
    extern  void Foo();
    
    // With this CL
    extern void Foo();
    
    Updates #43591.
    
    Change-Id: Ic2c21f8d806fe35a7be7183dbfe35ac605b6e4f6
    Reviewed-on: https://go-review.googlesource.com/c/go/+/283892
    Reviewed-by: Ian Lance Taylor <iant@golang.org>
    Trust: Katie Hockman <katie@golang.org>
    Reviewed-on: https://go-review.googlesource.com/c/go/+/300694
    Trust: Dmitri Shuralyov <dmitshur@golang.org>
    Trust: Emmanuel Odeke <emmanuel@orijtech.com>
    Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: David Chase <drchase@google.com>