Fix recursive loop on architectures where flags register is a dynamic value

[hugsy]

Description

Quick fix for a nasty bug I discovered last night:

For (at least) ARM and ARM64$arch.ptrsize is calculated dynamically from CSPR flags, and invokes parse_address. However, parse_address requires ptrsize to be set to determine the alignment.
So we end up in an infinite loop. Which breaks almost everything.

Example on ARM64

↳ File "/home/hugsy/code/gef/gef.py", line 3844, in parse_address()
    →     return to_unsigned_long(gdb.parse_and_eval(address))
↳ File "/home/hugsy/code/gef/gef.py", line 2351, in __get_register_for_selected_frame()
    →     return parse_address(regname)
↳ File "/home/hugsy/code/gef/gef.py", line 2345, in __get_register()
    →     return self.__get_register_for_selected_frame(regname, int(key))
↳ File "/home/hugsy/code/gef/gef.py", line 2363, in register()
    →     return self.__get_register(name)
↳ File "/home/hugsy/code/gef/gef.py", line 2584, in cpsr()
    →     return gef.arch.register(self.flag_register)
↳ File "/home/hugsy/code/gef/gef.py", line 2745, in is_aarch32()
    →     return (self.cpsr & (1 << 4) != 0) and (self.cpsr & (1 << 5) == 0)
↳ File "/home/hugsy/code/gef/gef.py", line 2756, in ptrsize()
    →     if self.is_aarch32():
↳ File "/home/hugsy/code/gef/gef.py", line 3411, in to_unsigned_long()
    →     mask = (1 << (gef.arch.ptrsize*8)) - 1
[...]

Repro: open any binary with gef on arm64, run start

Fix: Simply replacing to_unsigned_long with int is perfectly fine for this situation, which is what the PR does.

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

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