Specify multiple architectures for Julia to precompile to (#2044)

* Specify multiple architectures for Julia to precompile to

For amd64 (x86_64), we should specify what specific targets the
precompilation should be done for.  If we don't specify it,
it's *only* done for the target of the host doing the compilation.
When the container runs on a host that's still x86_64, but
a *different* generation of CPU than what the build host was, the
precompilation is useless and Julia takes a long long time to start
up. This specific multitarget comes from
https://docs.julialang.org/en/v1/devdocs/sysimg/#Specifying-multiple-system-image-targets,
and is the same set of options that the official Julia x86_64 build is
compiled with.  If the architecture the container runs on is
different, precompilation may still have to be re-done on first
startup - but this *should* catch most of the issues.

h/t to
https://discourse.julialang.org/t/is-it-possible-to-make-precompilation-portable-for-docker-images-built-with-a-different-cpu/95913
which helped point me towards `JULIA_CPU_TARGET`.

Fixes #2015 for more information

* Fix bash syntax issue

* Add JULIA_CPU_TARGET for aarch64 as well

- Don't need `export` as this is only used within this
  script
- Steal from upstream what should be setup for aarch64

* Re-add export for JULIA_CPU_TARGET

Quietens pre-commit

images/minimal-notebook/setup-scripts/setup-julia-packages.bash

@@ -5,6 +5,28 @@ set -exuo pipefail
 # - The JULIA_PKGDIR environment variable is set
 # - Julia is already set up, with the setup-julia.bash command
 
+
+# If we don't specify what CPUs the precompilation should be done for, it's
+# *only* done for the target of the host doing the compilation.  When the
+# container runs on a host that's the same architecture, but a *different*
+# generation of CPU than what the build host was, the precompilation is useless
+# and Julia takes a long long time to start up. This specific multitarget comes
+# from https://github.com/JuliaCI/julia-buildkite/blob/70bde73f6cb17d4381b62236fc2d96b1c7acbba7/utilities/build_envs.sh#L20-L76,
+# and may need to be updated as new CPU generations come out.
+# If the architecture the container runs on is different,
+# precompilation may still have to be re-done on first startup - but this
+# *should* catch most of the issues.  See
+# https://github.com/jupyter/docker-stacks/issues/2015 for more information
+if [ "$(uname -m)" == "x86_64" ]; then
+    # See https://github.com/JuliaCI/julia-buildkite/blob/70bde73f6cb17d4381b62236fc2d96b1c7acbba7/utilities/build_envs.sh#L24
+    # for an explanation of these options
+    export JULIA_CPU_TARGET="generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)"
+elif [ "$(uname -m)" == "aarch64" ]; then
+    # See https://github.com/JuliaCI/julia-buildkite/blob/70bde73f6cb17d4381b62236fc2d96b1c7acbba7/utilities/build_envs.sh#L54
+    # for an explanation of these options
+    export JULIA_CPU_TARGET="generic;cortex-a57;thunderx2t99;carmel"
+fi
+
 # Install base Julia packages
 julia -e '
 import Pkg;