aarch64: Bump LLVM version

12 jobs for !24 with aarch64 in 10 minutes and 1 second
merge request
Name Stage Failure
build-aarch64-linux-deb9 Build
<no location info>: error:
Warning: Couldn't figure out LLVM version!
Make sure you have installed LLVM 6.0
ghc: could not execute: opt

Error during cabal-install bootstrap:
Compiling the Setup script failed.
The command '/bin/bash -o pipefail -c EXTRA_CONFIGURE_OPTS=--disable-optimization ./bootstrap.sh --global --no-doc' returned a non-zero code: 2
ERROR: Job failed: exit code 2