Eliminate TSan false positive for stop_info.stack_ptr (v2)

Eliminate TSan false positive for stop_info.stack_ptr (v2)

Without this patch, Thread Sanitizer reports a data race between
GC_has_other_debug_info() and the code which sets stop_info.stack_ptr.

* include/private/pthread_support.h [THREAD_SANITIZER]: Include
dbg_mlc.h file.
* include/private/pthread_support.h [THREAD_SANITIZER] (GC_Thread_Rep):
Add dummy field (as the first field of the structure).
* pthread_support.c [THREAD_SANITIZER && CPPCHECK] (GC_new_thread):
Call GC_noop1(first_thread.dummy[0]) (to suppress cppcheck warning
about unused field).