Reenable crossgen2smoke under GC stress (#40849)

Reenable crossgen2smoke under GC stress (#40849)

This change removes the open-coded Crossgen2 execution
from the crossgen2smoke test project and switches it over
to use the normal CLRTest.Crossgen.targets logic for Crossgen2
invocation.

As the CLRTest.Crossgen.targets script already contains my fix
for the underlying issue - we were inadvertently running the
Crossgen2 compiler itself in GC stress mode and timing out
due to the long compilation times - we can now reenable
this test in GC stress mode.

Thanks

Tomas