scripts/gdb: add utils.read_ulong()

Add a function for reading unsigned long values, which vary in size
depending on the architecture.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200814212525.6118-2-john.ogness@linutronix.de