stdlib: Fix get_supported_isas.

This was equating the presence of the USE_*_ISA variables with the
support of that ISA, without actually checking that variable's value.

Change-Id: I4a19a694fa808de4d962ae2f5b30d9981b036224
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62200
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/python/gem5/runtime.py

@@ -46,7 +46,7 @@ def get_supported_isas() -> Set[ISA]:
         supported_isas.add(get_isa_from_str(buildEnv["TARGET_ISA"]))
 
     for key in get_isas_str_set():
-        if f"USE_{key.upper()}_ISA" in buildEnv:
+        if buildEnv.get(f"USE_{key.upper()}_ISA", False):
             supported_isas.add(get_isa_from_str(key))
 
     return supported_isas