Fix 'unknown attribute no_sanitize' compiler warnings for GCC

Fix 'unknown attribute no_sanitize' compiler warnings for GCC
(fix commits ce75cf1, b850e1c)

* include/private/gc_priv.h [!GC_ATTR_NO_SANITIZE_ADDR
&& ADDRESS_SANITIZER && !AO_CLANG_PREREQ(3,8)]
(GC_ATTR_NO_SANITIZE_ADDR): Define to
__attribute__((no_sanitize_address)).
* include/private/gc_priv.h [!GC_ATTR_NO_SANITIZE_MEMORY
&& MEMORY_SANITIZER && !AO_CLANG_PREREQ(3,8)]
(GC_ATTR_NO_SANITIZE_MEMORY): Define to
__attribute__((no_sanitize_memory)).
* include/private/gc_priv.h [!GC_ATTR_NO_SANITIZE_THREAD
&& THREAD_SANITIZER && !AO_CLANG_PREREQ(3,8)]
(GC_ATTR_NO_SANITIZE_THREAD): Define to
__attribute__((no_sanitize_thread)).