os: Implement Pipe for darwin, add smoke test.

Evidently when you read from pipes in Go, you have to adjust your slice length to reflect the number of bytes read.
Verified upstream behaves the same way.

src/syscall/syscall_libc_wasi.go

@@ -292,6 +292,10 @@ func Lstat(path string, p *Stat_t) (err error) {
 	return
 }
 
+func Pipe2(p []int, flags int) (err error) {
+	return ENOSYS // TODO
+}
+
 // int stat(const char *path, struct stat * buf);
 //export stat
 func libc_stat(pathname *byte, ptr unsafe.Pointer) int32