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>

src/python/gem5/components/processors/cpu_types.py

@@ -39,7 +39,7 @@ class CPUTypes(Enum):
     MINOR = "minor"
 
 
-def get_cpu_types_str_set() -> Set[CPUTypes]:
+def get_cpu_types_str_set() -> Set[str]:
     """
     Returns a set of all the CPU types as strings.
     """

src/python/gem5/isas.py

@@ -57,7 +57,7 @@ class ISA(Enum):
     NULL = "null"
 
 
-def get_isas_str_set() -> Set[ISA]:
+def get_isas_str_set() -> Set[str]:
     """
     Returns a set of all the ISA as strings.
     """