[sgen] Fix mono_gc_weak_link_get ().
[sgen] Binary protocol for disappearing link updates.
[gc] Remove mono_gc_disable/enable().

We don't use it anymore, and SGen doesn't honor it anyway.
[sgen] Fix GC handle table resize for SGen.  Fixes #10127.

SGen doesn't honor mono_gc_disable(), so the code was racy.