[lldb/test] Fix TestScriptedProcess.py timeout on x86_64

This patch fixes a timeout issue on the ScriptedProcess test that was
happening on intel platforms. The timeout was due to a misreporting of
the StopInfo in the ScriptedThread that caused the ScriptedProcess to
never stop.

To solve this, this patch changes the way a ScriptedThread reports its
stop reason by making it more architecture specific. In order to do so,
this patch also refactors the ScriptedProcess & ScriptedThread
initializer methods to provide an easy access to the target architecture.

Differential Revision: https://reviews.llvm.org/D118484



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>