cmd/internal/obj/mips: don't emit spurious CALLIND relocations

Generate a CALLIND relocation only for indirect calls, not for
indirect jumps. In particular, the RET instruction is lowered to
JMP (LR), an indirect jump, and occurs frequently. The large
amount of spurious relocations causes the linker to do a lot of
extra work.

Change-Id: Ie0edc04609788f5a687fd00c22558c3f83867697
Reviewed-on: https://go-review.googlesource.com/c/go/+/227079


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>