unix: add MemfdSecret on linux/riscv64

Now that SYS_MEMFD_SECRET is defined on linux/riscv64, CL 398514 can be
reverted to provide MemfdSecret on that platform as well.

Fixes golang/go#51547

Change-Id: I48e3e029fd2a3aeb860d69faf387bc10753759c7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/411376
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: tklauser

unix/syscall_linux_riscv64.go

@@ -22,6 +22,7 @@ import "unsafe"
 //sysnb	Getrlimit(resource int, rlim *Rlimit) (err error)
 //sysnb	Getuid() (uid int)
 //sys	Listen(s int, n int) (err error)
+//sys	MemfdSecret(flags int) (fd int, err error)
 //sys	pread(fd int, p []byte, offset int64) (n int, err error) = SYS_PREAD64
 //sys	pwrite(fd int, p []byte, offset int64) (n int, err error) = SYS_PWRITE64
 //sys	Seek(fd int, offset int64, whence int) (off int64, err error) = SYS_LSEEK