cmd/cgo/internal/testcarchive: build on all platforms

This test package uses syscall.SIGSEGV and syscall.SIGPIPE, which are
defined on most, but not all platforms. Normally this test runs as
part of dist test, which only registers this test on platforms that
support c-archive build mode, which includes all platforms that define
these signals. But this doesn't help if you're just trying to type
check everything in cmd.

Add build constraints so that this package type checks on all
platforms.

Fixes #60164.
Updates #37486.

Change-Id: Id3f9ad4cc9f80146de16aedcf85d108a77215ae6
Reviewed-on: https://go-review.googlesource.com/c/go/+/494659
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/cgo/internal/testcarchive/carchive_test.go

@@ -2,6 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// This test uses various syscall.SIG* constants that are defined on Unix
+// platforms and Windows.
+
+//go:build unix || windows
+
 package carchive_test
 
 import (