deThreadUnix: add __NR_sched_getaffinity macro definition check

[FASTSHIFT]

The NuttX platform does not support __NR_sched_getaffinity; a macro definition has been added to fall back to single-core mode.

CC:  VK-GL-CTS/framework/delibs/dethread/unix/deThreadUnix.c VK-GL-CTS/framework/delibs/dethread/unix/deThreadUnix.c: In function 'deGetNumAvailableLogicalCores':
VK-GL-CTS/framework/delibs/dethread/unix/deThreadUnix.c:175:11: warning: implicit declaration of function 'syscall'; did you mean 'sys_call0'? [-Wimplicit-function-declaration]
  175 |     ret = syscall(__NR_sched_getaffinity, 0, maskSize, &mask);
      |           ^~~~~~~
      |           sys_call0
VK-GL-CTS/framework/delibs/dethread/unix/deThreadUnix.c:175:19: error: '__NR_sched_getaffinity' undeclared (first use in this function); did you mean 'sched_getaffinity'?
  175 |     ret = syscall(__NR_sched_getaffinity, 0, maskSize, &mask);
      |                   ^~~~~~~~~~~~~~~~~~~~~~
      |                   sched_getaffinity
VK-GL-CTS/framework/delibs/dethread/unix/deThreadUnix.c:175:19: note: each undeclared identifier is reported only once for each function it appears in