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>

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

unix/zsyscall_linux_riscv64.go

@@ -180,6 +180,17 @@ func Listen(s int, n int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func MemfdSecret(flags int) (fd int, err error) {
+	r0, _, e1 := Syscall(SYS_MEMFD_SECRET, uintptr(flags), 0, 0)
+	fd = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func pread(fd int, p []byte, offset int64) (n int, err error) {
 	var _p0 unsafe.Pointer
 	if len(p) > 0 {