stdlib: Fix incorrect return type in cpu_types and isa

`get_cpu_types_str_set()` and `get_isas_str_set()` return `Set[str]` not
`Set[CPUTypes]`/`Set[ISA]`.

Change-Id: I703ce4c19e77eb6a3931cabb759f25d28aabb412
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61773
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
2 files changed