execabs: add legacy "//+build" constraints

Go releases before Go 1.17 are no longer supported, and Go 1.17
supports the newer "//go:build" syntax, so the older "//+build"
variation is no longer required by Go's support policy. However, the
x/sys module is very low-level and empirically some users are still on
unsupported Go releases, so there is little harm in adding the older
variations.

Fixes golang/go#52668.

Change-Id: Ieb8e64e450b9d795f2753fc5f99b36816a3dbff3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/403695
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

execabs/execabs_go118.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 //go:build !go1.19
+// +build !go1.19
 
 package execabs
 

execabs/execabs_go119.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.19
+// +build go1.19
 
 package execabs