Can't cross compile via buildx for other architectures

[vennb]

NGINX Ingress controller version: 0.30.0

Kubernetes version N/A

Environment: Build attempt on Ubuntu 18.04

• Cloud provider or hardware configuration: IBM Cloud
• OS 18.04.2 LTS (Bionic Beaver)
• Kernel (e.g. uname -a): Linux virtualserver01 4.15.0-54-generic Fix "invalid port in upstream" on nginx controller #58-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux
• Install tools: N/A
• Others:

What happened:

Attempting to build igress-nginx for s390x.

When building the nginx image under

/ingress-nginx/images/nginx

Added in the Makefile

PLATFORMS = s390x

Running

make

Eventually breaks with

#7 5714. chmod +rx ./luarocks-admin
#7 5714. rm -f src/luarocks/core/hardcoded.lua
#7 5714. echo "#!/bin/sh" > luarocks
#7 5714. echo "unset LUA_PATH LUA_PATH_5_2 LUA_PATH_5_3 LUA_PATH_5_4" >> luarocks
#7 5714. echo 'LUAROCKS_SYSCONFDIR="/usr/local/etc/luarocks" LUA_PATH="/tmp/build/luarocks-3.1.3/src/?.lua;;" exec "/usr/local/bin/luajit" "/tmp/build/luarocks-3.1.3/src/bin/luarocks" --project-tree="/tmp/build/luarocks-3.1.3/lua_modules" "$@"' >> luarocks
#7 5714. chmod +rx ./luarocks
#7 5714. ./luarocks init
#7 5715. make: *** [GNUmakefile:55: luarocks] Illegal instruction (core dumped)
#7 5715. make: *** Deleting file 'luarocks'

**What you expected to happen**:

Looks like during the cross build process, it is trying to run luarocks, which won't work as although Docker supports cross compiling for different using buildx, it doesn't support running on different architectures

**How to reproduce it**:
As above, try building the nginx image using buildx for a different architecture 

## Install the ingress controller

N/A

## Install an application that will act as default backend (is just an echo app)

N/A

## Create an ingress (please add any additional annotation required)

N/A

## make a request

N/A

**Anything else we need to know**:

/kind bug

[aledbf]

Attempting to build igress-nginx for s390x.

Closing. The platform s390x is not supported. Only amd64, arm, and arm64 are.

[aledbf]

@vennb experimental support for s390x was added in release 0.33.0.