[lldb] Fix failure in TestStackCoreScriptedProcess on x86_64

This patch should address the failure of TestStackCoreScriptedProcess
that is happening specifically on x86_64.

It turns out that in 1370a1cb, I changed the way we extract integers
from a `StructuredData::Dictionary` and in order to get a stop info from
the scripted process, we call a method that returns a `SBStructuredData`
containing the stop reason data.

TestStackCoreScriptedProcess` was failing specifically on x86_64 because
the stop info dictionary contains the signal number, that the `Scripted
Thread` was trying to extract as a signed integer where it was actually
parsed as an unsigned integer. That caused `GetValueForKeyAsInteger` to
return the default value parameter, `LLDB_INVALID_SIGNAL_NUMBER`.

This patch address the issue by extracting the signal number with the
appropriate type and re-enables the test.

Differential Revision: https://reviews.llvm.org/D152848



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>