runtime: run TestCgoSigfwd on all Unix platforms

This test was originally Linux-only, but there doesn't seem to be
anything Linux-specific in it.

Change-Id: I0f8519eff5dbed97f5e21e1c8e5ab0d747d51df3
Reviewed-on: https://go-review.googlesource.com/c/go/+/443073
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/goos/nonunix.go

@@ -0,0 +1,5 @@
+//go:build !unix
+
+package goos
+
+const IsUnix = false

src/internal/goos/unix.go

@@ -0,0 +1,5 @@
+//go:build unix
+
+package goos
+
+const IsUnix = true

src/runtime/crash_cgo_test.go

@@ -757,8 +757,8 @@ func TestCgoTraceParserWithOneProc(t *testing.T) {
 
 func TestCgoSigfwd(t *testing.T) {
 	t.Parallel()
-	if goos.IsLinux == 0 {
-		t.Skipf("only supported on Linux")
+	if !goos.IsUnix {
+		t.Skipf("no signals on %s", runtime.GOOS)
 	}
 
 	got := runTestProg(t, "testprogcgo", "CgoSigfwd", "GO_TEST_CGOSIGFWD=1")

src/runtime/testdata/testprogcgo/sigfwd.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux
+//go:build unix
 
 package main