[gdb] allow disable of target-async #795
Description
We needed to revert #794
due to it causing gdb to hang in certain instances when async not set to on.
Repro: With it not set to on, if you debug android in VS and while app is running, attempt to terminate, it causes VS to hang. This is because android gdb still supports async even though the list-target-features doesn't return it.
Option: enable via json/xml option.