cmd/cgo/internal/testsanitizers: build on all platforms
This test package uses the Pdeathsig field of syscall.SysProcAttr,
which is only available on a few platforms. Currently, dist test
checks for compatible platforms and only registers it as part of
all.bash on platforms where it can build. But this doesn't help if
you're just trying to type check everything in cmd.
Make this package pass type checking by moving the condition from dist
into build tags on the test package itself.