unix: add unexported name_to_handle_at and open_by_handle_at types & wrappers

No usable change for users in this CL; just auto-generated syscall
wrapper funcs & types.

The next CL will have hand-written code adding the nice Go API around
these unexported symbols. (as outlined in the comment at
https://golang.org/issue/30537#issuecomment-470284573)

Updates golang/go#30537

Change-Id: I5e34df517efcf509fff97f670425500cc6608d59
Reviewed-on: https://go-review.googlesource.com/c/sys/+/172584
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux.go

@@ -1662,6 +1662,19 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 	return EACCES
 }
 
+//sys nameToHandleAt(dirFD int, pathname string, fh *fileHandle, mountID *_C_int, flags int) (err error) = SYS_NAME_TO_HANDLE_AT
+//sys openByHandleAt(mountFD int, fh *fileHandle, flags int) (fd int, err error) = SYS_OPEN_BY_HANDLE_AT
+
+// fileHandle is the argument to nameToHandleAt and openByHandleAt. We
+// originally tried to generate it via unix/linux/types.go with "type
+// fileHandle C.struct_file_handle" but that generated empty structs
+// for mips64 and mips64le. Instead, hard code it for now (it's the
+// same everywhere else) until the mips64 generator issue is fixed.
+type fileHandle struct {
+	Bytes uint32
+	Type  int32
+}
+
 /*
  * Unimplemented
  */