syscall: fix riscv64 GNU/Linux build

Make syscall_linux_riscv64.go, new in the 1.14beta1 release, look like
the other syscall_linux_GOARCH.go files.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/217577

gcc/go/gofrontend/MERGE

-d796680b5a78f686ed118578e81d5b1adf48508d
+c94637ad6fd38d4814fb02d094a1a73f19323d71
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/syscall_linux_riscv64.go

@@ -4,20 +4,6 @@
 
 package syscall
 
-import "unsafe"
-
 func (r *PtraceRegs) PC() uint64 { return r.Pc }
 
 func (r *PtraceRegs) SetPC(pc uint64) { r.Pc = pc }
-
-func (iov *Iovec) SetLen(length int) {
-	iov.Len = uint64(length)
-}
-
-func (msghdr *Msghdr) SetControllen(length int) {
-	msghdr.Controllen = uint64(length)
-}
-
-func (cmsg *Cmsghdr) SetLen(length int) {
-	cmsg.Len = uint64(length)
-}