Workaround TSan warning about data race in generic_malloc_many

Workaround TSan warning about data race in generic_malloc_many

It is acceptable to update GC_bytes_found counter without
synchronization (so, it is OK to have inaccurate value in
GC_bytes_found in favor of performance).

* mallocx.c [PARALLEL_MARK && THREAD_SANITIZER]
(GC_generic_malloc_many): Acquire the allocation lock to update
GC_bytes_found (after successful GC_reclaim_generic).
* mallocx.c [PARALLEL_MARK && !THREAD_SANITIZER]
(GC_generic_malloc_many): Move GC_bytes_found update into the block
executed with the mark lock held.