TSan reports data races in some functions of the debug runtime

[OlivierNicole]

Running the testsuite on trunk revision 5348fa9 with the debug runtime and TSan enabled reports 4 data races. They are in functions that are run only by the debug runtime, such as caml_verify_heap_from_stw or verify_pool.

• Race on a static local variable in scan_native_globals called from caml_verify_heap_from_stw in test lib-format/mc_pr586_par.ml https://gist.github.com/OlivierNicole/72afa15c639980f158a63643a2bbf014
• do_some_marking / verify_poolin lib-runtime-events/test_dropped_events.ml https://gist.github.com/OlivierNicole/75926b7475d593a1f73fdcb1b12aa131
• caml_darken_cont / verify_pool in parallel/mctest.ml https://gist.github.com/OlivierNicole/a2946916685cead844cbbcf3beee5110
• caml_darken / verify_pool in parallel/mctest.ml https://gist.github.com/OlivierNicole/954630de207ee9c77ce4796824ecf89e

[OlivierNicole]

Correction: the first race in scan_native_globals may impact the normal runtime as well.