[sgen] Remove weak ref hashes.
[sgen] Make GC lock recursive.
[sgen] Use GC lock for locking weak ref table.
[sgen] Deduplicate 'gc_handles' initializers.
[sgen] Remove magic numbers.
[sgen] Refactor 'alloc_handle()'.
[sgen] Avoid redundant calls in 'mono_gchandle_iterate()'.
[sgen] Remove magic numbers in 'gc.c'.