Use noexcept in gc_allocator

Use noexcept in gc_allocator

* include/gc_allocator.h [!GC_NOEXCEPT] (GC_NOEXCEPT): Define internal
macro (to noexcept if C++11, otherwise to throw() or nothing); define
GC_NEW_ABORTS_ON_OOM if GC_NOEXCEPT is defined to nothing.
* include/gc_allocator.h (gc_allocator(), deallocate, operator==,
operator!=, gc_allocator_ignore_off_page()): Add GC_NOEXCEPT.
* include/gc_allocator.h (gc_allocator, gc_allocator_ignore_off_page,
traceable_allocator): Replace throw() to GC_NOEXCEPT.
* include/gc_allocator.h (deallocate): Remove outdated comment that
p should be non-null.