cmd/internal/obj: fix tail call in non-zero frame leaf function on MIPS and S390X

A "RET f(SB)" wasn't assembled correctly in a leaf function with
non-zero frame size. Follows CL 371034, for MIPS(32/64)(be/le)
and S390X. Other architectures seem to do it right. Add a test.

Change-Id: I41349a7ae9862b924f3a3de2bcb55b782061ce21
Reviewed-on: https://go-review.googlesource.com/c/go/+/371214


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>