TSAN failures in exitTicker

Currently the non-threaded RTS produces spurious ThreadSanitizer failures of the form:

WARNING: ThreadSanitizer: data race (pid=118233)
  Write of size 1 at 0x7f66710635f0 by main thread:
    #0 exitTicker rts/posix/ticker/Pthread.c:246 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000007ffc8)
    #1 exitTimer rts/Timer.c:209 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000005b4f7)
    #2 hs_exit_ rts/RtsStartup.c:478 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000004ccb0)
    #3 shutdownHaskellAndExit rts/RtsStartup.c:640 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000004d375)
    #4 hs_main rts/RtsMain.c:99 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000004c259)
    #5 main <null> (ghc-pkg-9.3.20211002+0x00000047a574)
  Previous read of size 1 at 0x7f66710635f0 by thread T1:
    #0 itimer_thread_func rts/posix/ticker/Pthread.c:128 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000007fb83)
    #1 <null> <null> (libtsan.so.0+0x000000024459)
  Location is global 'exited' of size 1 at 0x7f66710635f0 (libHSrts-1.0.2-ghc9.3.20211002.so+0x0000000b35f0)
  Thread T1 'ghc_ticker' (tid=118235, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x000000027e9d)
    #1 initTicker rts/posix/ticker/Pthread.c:202 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000007fe1b)
    #2 initTimer rts/Timer.c:180 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000005b0fa)
    #3 hs_init_ghc rts/RtsStartup.c:295 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000004cf98)
    #4 hs_main rts/RtsMain.c:57 (libHSrts-1.0.2-ghc9.3.20211002.so+0x00000004c1eb)
    #5 main <null> (ghc-pkg-9.3.20211002+0x00000047a574)
SUMMARY: ThreadSanitizer: data race rts/posix/ticker/Pthread.c:246 in exitTicker

This is because the non-threaded RTS compiles out all of the barrier and atomic operations defined in rts/include/stg/SMP.h. Given that the non-threaded RTS is now no longer single-threaded (as it may use the pthread ticker implementation) we should probably compile at least some of these as their threaded implementations.