[lldb/crashlog] Fix arm64 register parsing on crashlog.py

This patch fixes the register parser for arm64 crashlogs.

Compared to x86_64 crashlogs, the arm64 crashlogs nests the general
purpose registers into a separate dictionary within `thread_state`
dictionary. It uses the dictionary key as the the register number.

Differential Revision: https://reviews.llvm.org/D119168



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