gc: fix build errors in gc-stock.c and gc-debug.c (#61627)

Fix missing `(JL_STREAM*)` cast for `jl_safe_static_show` in
`gc_assert_parent_validity` in gc-stock.c, which caused an
incompatible pointer type error.

Fix stale initializer in gc-debug.c: the `always_full` field was
removed from `jl_gc_debug_env_t` in #58713 but the corresponding `0`
in the initializer was not removed, causing excess-elements-in-scalar
warnings.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>