Subtract sizeof LOSObject instead of LOSObject*

On 64-bit mono with marksweep-conc enabled, sgen_los_pin_object()
would mistakenly set the cardtable_mod_union pointer field to 1
due to the incorrect pointer math in sgen_los_header_for_object()
Merge pull request #586 from awinters-fvs/awinters/sgen-los-sizeof-fix

[sgen] marksweep-conc 64-bit pointer arithmetic fix for segfault