Fix correct accumulated time for parallelized jobs in SGEN. (mono/mono#17518)

Fix correct accumulated time for parallelized jobs in SGEN. (mono/mono#17518)

* Fix correct accumulated time for parallelized jobs in SGEN.

Current implementation incorrectly accumulate time for scan jobs for
the follow metrics when running parallel minor GC:

time_minor_scan_major_blocks,
time_minor_scan_los,
time_major_scan_mod_union_blocks,
time_major_scan_mod_union_los

Commit fix so updates are atomic making sure values are correct in cases
where parallel minor GC is used.

* Implement 64-bit atommic add fallback in SGEN.

Commit migrated from https://github.com/mono/mono/commit/44e6226c31d8ffcae58f81350d71a728edecfe22