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.