cmd/internal/obj/arm64: use more appropriate types for olsr9s/olsr12u
This allows for a large number of casts to be removed at call sites.
While here, use consistent register naming.
Change-Id: I78a2a928b78c9f09f91fb6ed6ad440aa4e63923d
Reviewed-on: https://go-review.googlesource.com/c/go/+/471517
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Michael Knyszek <mknyszek@google.com>