[coop handles] Add some memory fences (#3617)

[coop handles] Add some memory fences (#3617)

When running with async suspend (ie, not coop), a user thread might be
suspended while its manipulating the coop handle stack.  In that case,
we want to present a consitent view to mono_handle_stack_scan () by
adding memory fences to code that updates the stack chunks.  In
particular we want to ensure that the chunk size is increased after
the new slot has a valid (null) value.  Also when creating new chunks
make sure their size is initialized before the chunk stack is updated.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=43921 (at least the
one stack trace observed in [comment 1](https://bugzilla.xamarin.com/show_bug.cgi?id=43921#c1)).