No setup information found for ghc-8.4.3 on centos (`linux64-gmp4`)

### General summary/comments (optional)

Currently getting this in my CentOS 6.9 based build:
```
Downloading lts-12.0 build plan ...
Downloaded lts-12.0 build plan.
No setup information found for ghc-8.4.3 on your platform.
This probably means a GHC bindist has not yet been added for OS key 'linux64-gmp4'.
Supported versions: ghc-7.8.4, ghc-7.10.2, ghc-7.10.3, ghc-8.0.1, ghc-8.0.2, ghc-8.2.1, ghc-8.2.2, ghc-8.4.2
```

### Steps to reproduce

`Dockerfile`:
```
FROM centos:6.9
## Install dependencies
RUN yum install -y curl\
gcc \
gmp-devel \
pcre-devel \
perl \
tar \
which \
xz \
zlib-devel \
bzip2-devel \
&& yum clean all --releasever=6 \
&& ln -s /lib64/libtinfo.so.5 /lib64/libtinfo.so

ENV GHC_VERSION=8.4.3

## Install stack
ARG STACK_VER=1.7.1
RUN mkdir -p /opt/stack/bin
RUN curl -L https://github.com/commercialhaskell/stack/releases/download/v$STACK_VER/stack-$STACK_VER-linux-x86_64-gmp4.tar.gz | tar xz --wildcards --strip-components=1 -C /opt/stack/bin '*/stack'
ENV PATH /opt/stack/bin/:$PATH
RUN stack setup $GHC_VERSION
```
run:
`docker build .`

### Expected

docker build to succeed and provide the GHC 8.4.3 inside the container

### Actual

see the error above

### Stack version

`https://github.com/commercialhaskell/stack/releases/download/v1.7.1/stack-1.7.1-linux-x86_64-gmp4.tar.gz`

### Method of installation

see the `Dockerfile` above

### Comments

looks like the GHC 8.4.3 bindist is missing for Centos 6
https://github.com/fpco/stackage-content/blob/6b816d9287b408e24c2aa0d19477fab0f5444e8b/stack/stack-setup-2.yaml#L369