gdb/python/python-internal.h: avoid uninitialized constexpr

The following recent change introduced a regression when building using
clang++:

    commit 764af878
    Date:   Wed Jun 12 18:58:49 2024 +0200

        [gdb/python] Add typesafe wrapper around PyObject_CallMethod

The error message is:

    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
      CXX    python/py-block.o
    1 error generated.
    make[2]: *** [Makefile:1959: python/py-arch.o] Error 1
    make[2]: *** Waiting for unfinished jobs....
    In file included from ../../gdb/python/py-auto-load.c:25:
    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
    1 error generated.
    make[2]: *** [Makefile:1959: python/py-auto-load.o] Error 1
    In file included from ../../gdb/python/py-block.c:23:
    ../../gdb/python/python-internal.h:151:16: error: default initialization of an object of const type 'const char'
    constexpr char gdbpy_method_format;
                   ^
                                       = '\0'
    1 error generated.

This patch fixes this by changing gdbpy_method_format to be a templated
struct, and only have its specializations define the static constexpr
member "format".  This way, we avoid having an uninitialized constexpr
expression, regardless of it being instantiated or not.

Reviewed-By: Tom de Vries <tdevries@suse.de>
Change-Id: I5bec241144f13500ef78daea30f00d01e373692d