[sgen] Improve fix for BXC#14834 to avoid redundant code

Improve https://github.com/mono/mono/commit/d2cc22580898df5d4a15e0f99ab513e1570a6082
by making OBJ_BITMAP_FOREACH be less redundant thanks to new macro
GNUC_BUILTIN_CTZ
Merge pull request #765 from knocte/runtime_fixage

[sgen] Improve fix for BXC#14834 to avoid redundant code