runtime: support GODEBUGCPU for more Unix-like operating systems

Adds AIX, DragonFly BSD, FreeBSD, NetBSD, OpenBSD and Solaris
to the list of operating systems where the GODEBUGCPU environment
variable will be parsed and interal/cpu features can be enabled
and disabled.

Updates #27218

Change-Id: I9cd99142e2a5147cb00ca57b581f049ea6ce8508
Reviewed-on: https://go-review.googlesource.com/c/145281Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -483,7 +483,8 @@ func cpuinit() {
 	const prefix = "GODEBUGCPU="
 	var env string
 
-	if GOOS == "linux" || GOOS == "darwin" {
+	switch GOOS {
+	case "aix", "darwin", "dragonfly", "freebsd", "netbsd", "openbsd", "solaris", "linux":
 		cpu.DebugOptions = true
 
 		// Similar to goenv_unix but extracts the environment value for