util: Rename DETECT_OS_UNIX to DETECT_OS_POSIX

Looking at each usage of DETECT_OS_UNIX, it's more about the POSIX API usage, not the
Unix-like OS, so let's rename it

And for POSIX it's a standard to claim which API present, but for UNIX there is no such thing

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29555>

src/util/u_cpu_detect.c

@@ -72,7 +72,7 @@
 #include <elf.h>
 #endif
 
-#if DETECT_OS_UNIX
+#if DETECT_OS_POSIX
 #include <unistd.h>
 #endif
 
@@ -736,7 +736,7 @@ _util_cpu_detect_once(void)
       GetSystemInfo(&system_info);
       available_cpus = MAX2(1, system_info.dwNumberOfProcessors);
    }
-#elif DETECT_OS_UNIX
+#elif DETECT_OS_POSIX
 #  if defined(HAS_SCHED_GETAFFINITY)
    {
       /* sched_setaffinity() can be used to further restrict the number of
@@ -804,7 +804,7 @@ _util_cpu_detect_once(void)
       total_cpus = ncpu;
    }
 #  endif /* DETECT_OS_BSD */
-#endif /* DETECT_OS_UNIX */
+#endif /* DETECT_OS_POSIX */
 
    util_cpu_caps.nr_cpus = MAX2(1, available_cpus);
    total_cpus = MAX2(total_cpus, util_cpu_caps.nr_cpus);