Workaround 'Uninitialized variable' cppcheck errors

Workaround 'Uninitialized variable' cppcheck errors

* extra/AmigaOS.c [GC_AMIGA_DS] (GC_register_data_segments): Initialize
myseglist outside conditional statement.
* mach_dep.c [GETCONTEXT_FPU_EXCMASK_BUG && X86_64]
(GC_with_callee_saves_pushed): Call GC_noop1(&old_fcw) (before asm
fstcw) if CPPCHECK.
* mach_dep.c [!HAVE_BUILTIN_UNWIND_INIT] (GC_with_callee_saves_pushed):
Replace regs with &regs.
* os_dep.c [!MSWIN32 && !GC_OPENBSD_THREADS && ...]
(GC_get_main_stack_base): Set result to NULL if CPPCHECK but none of
HEURISTIC* and *STACKBOTTOM defined.