[gdb/python] Fix gdb.python/py-disasm.exp on arm-linux

After fixing test-case gdb.python/py-disasm.exp to recognize the arm nop:
...
	nop	{0}
...
we run into:
...
disassemble test^M
Dump of assembler code for function test:^M
   0x004004d8 <+0>:     push    {r11}           @ (str r11, [sp, #-4]!)^M
   0x004004dc <+4>:     add     r11, sp, #0^M
   0x004004e0 <+8>:     nop     {0}^M
=> 0x004004e4 <+12>:    Python Exception <class 'ValueError'>: Buffer \
  returned from read_memory is sized 0 instead of the expected 4^M
^M
unknown disassembler error (error = -1)^M
(gdb) FAIL: $exp: global_disassembler=ShowInfoRepr: disassemble test
...

This is caused by this code in gdbpy_disassembler::read_memory_func:
...
  gdbpy_ref<> result_obj (PyObject_CallMethod ((PyObject *) obj,
                                              "read_memory",
                                              "KL", len, offset));
...
where len has type "unsigned int", while "K" means "unsigned long long" [1].

Fix this by using "I" instead, meaning "unsigned int".

Also, offset has type LONGEST, which is typedef'ed to int64_t, while "L" means
"long long".

Fix this by using type gdb_py_longest for offset, in combination with format
character "GDB_PY_LL_ARG".  Likewise in disasmpy_info_read_memory.

Tested on arm-linux.

Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>

PR python/31845
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31845

[1] https://docs.python.org/3/c-api/arg.html

(cherry picked from commit 4cd214dc)