For x86_64, in mono_gc_make_root_descr_all_refs when numbits=32 gc_bitmap contains:
ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00

In mono_gc_make_descr_from_bitmap bitmap[0] looks like this -
ff ff ff ff 00 00 00 00

And after the shift it becomes -
00000007fffffff9

Whereas for s390x we see:

gc_bitmap:
ff ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00

bitmap[0]:
ff ff ff ff 00 00 00 00

Value after shift:
0xfffffff800000001