Disable LTO on non-amd64 builds

Coroutines fail with LTO on non-amd64 architectures, see:

• https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1921664
• https://bugzilla.redhat.com/show_bug.cgi?id=1952483

Debian doesn't enable LTO by default yet, but this is likely to happen at some point [1]. For the moment this fixes Debian builds when LTO is manually enabled and builds in downstream distros where LTO is enabled by default (e.g. Ubuntu).

[1] https://lists.debian.org/debian-devel/2022/06/msg00092.html

LP: #1921664