transform sgen_get_descriptor to parallel safe version in job_major_mod_union_preclea...

transform sgen_get_descriptor to parallel safe version in job_major_mod_union_preclean (#63293)

Fixes mono/mono#21369
Related to https://github.com/xamarin/xamarin-android/issues/6546

job_major_mod_union_preclean can race with the tarjan bridge
implementation that changes the vtable pointer by settings the three
lower bits. this results in invalid loading of the vtable
(shifted by 7 bytes)  which in turn give a wrong desc to the scan
functions

This change is released under the MIT license.

Co-authored-by: tmijieux <tmijieux@users.noreply.github.com>