util: Enable using different call types in the gem5 ops java wrapper.

util: Enable using different call types in the gem5 ops java wrapper.

Convert the native implementation from C to C++. Also expand the test to
cycle through the different call mechanisms and call "sum" using each
one. This test should primarily be run on a gem5 native CPU which will
support all call types.

To access a particular call type, get an instance of the gem5.Ops class
from the callTypes static map, using the name of the call type you want
as the key. If you just want whatever the default is, use the additional
key "default".

Change-Id: If4ef812c9746fbf974e54cc9fe515e2b581e9939
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28182
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>