cmd/compile/internal/gc: fix return value offset for SSA backend on ARM

Progress on SSA backend for ARM. Still not complete. It compiles a
Fibonacci function, but the caller picked the return value from an
incorrect offset. This CL adjusts it to match the stack frame layout
for architectures with link register.

Updates #15365.

Change-Id: I01e03c3e95f5503a185e8ac2b6d9caf4faf3d014
Reviewed-on: https://go-review.googlesource.com/22186Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -2668,7 +2668,7 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 		return nil
 	}
 	fp := res.Field(0)
-	return s.entryNewValue1I(ssa.OpOffPtr, Ptrto(fp.Type), fp.Offset, s.sp)
+	return s.entryNewValue1I(ssa.OpOffPtr, Ptrto(fp.Type), fp.Offset+Ctxt.FixedFrameSize(), s.sp)
 }
 
 // etypesign returns the signed-ness of e, for integer/pointer etypes.