[gdb/guile] Use SCM_DEBUG_TYPING_STRICTNESS 0

[gdb/guile] Use SCM_DEBUG_TYPING_STRICTNESS 0

I build gdb with libguile v2.0.9, and ran into:
...
In file included from /usr/include/guile/2.0/libguile.h:56,
                 from ../../gdb/guile/guile-internal.h:30,
                 from ../../gdb/guile/scm-arch.c:26:
/usr/include/guile/2.0/libguile/inline.h: In function 'int scm_is_pair(SCM)':
/usr/include/guile/2.0/libguile/tags.h:97:53: error: \
  operation on '*0' may be undefined [-Werror=sequence-point]
 #   define SCM_UNPACK(x) ((scm_t_bits) (0? (*(SCM*)0=(x)): x))
                                            ~~~~~~~~~^~~~~
...

Fix this by using SCM_DEBUG_TYPING_STRICTNESS 0.

We were already using this for c++20 due to a Werror=volatile in SCM_UNPACK
when using libguile v2.0.10.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>