[gdb/testsuite] Fix gdb.python/py-format-string.exp with -m32

[gdb/testsuite] Fix gdb.python/py-format-string.exp with -m32

When running test-case gdb.python/py-format-string.exp with target board
unix/-m32, we run into:
...
(gdb) python print \
  (gdb.parse_and_eval ('a_base_ref').format_string (deref_refs=True))^M
@0xffffc468: {_vptr.Base = 0x80487e0 <vtable for Deriv+8>, a = 42, \
              static a_static_member = 2019}^M
(gdb) FAIL: gdb.python/py-format-string.exp: format_string: \
  lang_cpp: a_base_ref with option deref_refs: deref_refs=true
...
while with -m64, we have instead:
...
@0x7fffffffd170: {_vptr.Base = 0x400910 <vtable for Deriv+16>, a = 42, \
                  static a_static_member = 2019}^M
(gdb) PASS: gdb.python/py-format-string.exp: format_string: \
  lang_cpp: a_base_ref with option deref_refs: deref_refs=true
...

The vtable contains pointer entries which are 4-byte for -m32 and 8-byte for
-m64, so it's not surprising the offsets (Deriv+8 vs. Deriv+16) differ.

Fix this by allow Deriv+$decimal.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-20  Tom de Vries  <tdevries@suse.de>

	* gdb.python/py-format-string.exp: Allow Deriv+$decimal as vtable
	offset.