[Morello] Fix purecap driver when targeting gnu/linux.

Select the correct multilib for purecap (gcc uses
purecap/c64).

Don't rely on a purecap environment in the triple for selecting
the dynamic linker/library folder name. Use the detected
ABI instead.
29 jobs for !222 with driver in 279 minutes and 6 seconds (queued for 10 seconds)
latest merge request