cmd/asm,cmd/compile: fix tail call in leaf functions on PPC64

In some leaf functions using "RET foo(SB)", the jump may be incorrectly
translated into "JMP LR" instead of "JMP foo(SB)".

Such is the case when compiling the autogenerated function in k8s
k8s.io/kubernetes/pkg/kubelet/server/stats.(*resourceAnalyzer).GetPodVolumeStats.

Fixes #50048

Change-Id: Icdf65fcda6b3c5eb9d3ad5c7aad04add9419dffb
Reviewed-on: https://go-review.googlesource.com/c/go/+/371034
Run-TryBot: Paul Murphy <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Trust: Paul Murphy <[email protected]>

Author: pmur

src/cmd/internal/obj/ppc64/obj9.go

@@ -884,8 +884,13 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				q = c.newprog()
 				q.As = ABR
 				q.Pos = p.Pos
-				q.To.Type = obj.TYPE_REG
-				q.To.Reg = REG_LR
+				if retTarget == nil {
+					q.To.Type = obj.TYPE_REG
+					q.To.Reg = REG_LR
+				} else {
+					q.To.Type = obj.TYPE_BRANCH
+					q.To.Sym = retTarget
+				}
 				q.Mark |= BRANCH
 				q.Spadj = +autosize