Correct OSSL_sleep for NonStop PUT model by introducing sleep().

This fix also removes SPT model support as it was previously deprecated.
Upcoming threading models on the platform should be supportable without change
to this method.

Fixes: #23923
Fixes: #23927
Fixes: #23928

Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #23926)

crypto/sleep.c

@@ -22,20 +22,11 @@ void OSSL_sleep(uint64_t millis)
     ts.tv_sec = (long int) (millis / 1000);
     ts.tv_nsec = (long int) (millis % 1000) * 1000000ul;
     nanosleep(&ts, NULL);
-# elif defined(__TANDEM)
-#  if !defined(_REENTRANT)
+# elif defined(__TANDEM) && !defined(_REENTRANT)
 #   include <cextdecs.h(PROCESS_DELAY_)>
 
     /* HPNS does not support usleep for non threaded apps */
     PROCESS_DELAY_(millis * 1000);
-#  elif defined(_SPT_MODEL_)
-#   include <spthread.h>
-#   include <spt_extensions.h>
-
-    usleep(millis * 1000);
-#  else
-    usleep(millis * 1000);
-#  endif
 # else
     unsigned int s = (unsigned int)(millis / 1000);
     unsigned int us = (unsigned int)((millis % 1000) * 1000);