[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>