feat: adds exec/target qemu toolchain configs

This is a superset of !93 (closed), adding toolchains to allow the correct qemu parameters to be used based on matching architectures.

One example change is to use the Hypervisors when the cpu architectures for target and exec match. E.g. using the -accel kvm and then attempting OS specific accelerators before falling back onto the default -accel tcg.

The accelerators that have been chosen were based on the table in the QEMU accelerator docs:

https://www.qemu.org/docs/master/system/introduction.html#virtualisation-accelerators

Another optimisation for amd64/arm64 target/exec combination is to use these additional qemu flags:

machine: "virt,virtualization=on"
cpu: "max,pauth-impdef=on"

relates to #6, #12 (closed)