Fix segfaults with debug mode code on 64-bit Arm

[SeanTAllen]

While getting pony working on 64-bit Arm, we found a bug where the
test-stdlib-debug tests would segfault. Everything worked fine with release
builds, but debug builds would crash.

After investigation, we've found that in codegen_machine in host.cc if the
opt_level is get to either CodeGenOpt::Default or CodeGenOpt::Aggressive
instead of CodeGenOpt::None the problem goes away.

It seems likely that this is an LLVM bug, but that hasn't been established.
We've committed a fix in host.cc that when the target is arm (which is
64-bit arm) that we use CodeGenOpt::Default. This is far from ideal, but gets
us working compilations until we can investigate further.

The end result of this should either be a patch that we upstream to LLVM or
possibly, the discovery that we are doing something wrong with our Arm code
generation that optimization manages to fix.

[ponylang-main]

Hi @SeanTAllen,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3875.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.