sched/tickless: Fix CLOCK_MONOTONIC always 0 by updating sched ticks

[anchao]

Summary

sched/tickless: Fix CLOCK_MONOTONIC always 0 by updating sched ticks

Fix the issue where clock_gettime(CLOCK_MONOTONIC) always returns 0 in
tickless mode, caused by the scheduler tick counter (g_system_ticks) not
being updated with the actual timer ticks.

1. Add clock_update_sched_ticks() function to sched/clock/clock_sched_ticks.c:
   a. This function directly sets the system tick counter to a specific value
   (vs incrementing in clock_increase_sched_ticks), with full documentation
   matching the existing code style.
   b. Uses seqlock to ensure thread-safe access to g_system_ticks.

2. Call clock_update_sched_ticks() in nxsched_process_timer() (sched_processtickless.c):

a. Syncs the scheduler tick counter with the actual timer ticks retrieved via up_timer_gettick().

With this fix, g_system_ticks is properly updated in tickless mode,
and clock_gettime(CLOCK_MONOTONIC) returns the correct non-zero monotonic time.

Signed-off-by: chao an anchao.archer@bytedance.com

Impact

sched/tickless

Testing

sim/nsh:

enable config:
+CONFIG_SCHED_TICKLESS=y
+CONFIG_SYSLOG_TIMESTAMP=y

print syslog in exmaple hello:

$ git diff .
diff --git a/examples/hello/hello_main.c b/examples/hello/hello_main.c
index fd194a623..c92ec0788 100644
--- a/examples/hello/hello_main.c
+++ b/examples/hello/hello_main.c
@@ -38,5 +38,6 @@
 int main(int argc, FAR char *argv[])
 {
   printf("Hello, World!!\n");
+  syslog(0, "Hello, World!!\n");
   return 0;
 }

Before this patch:

nsh> hello
Hello, World!!
[    0.000000] Hello, World!!
nsh> hello
Hello, World!!
[    0.000000] Hello, World!!
nsh> 

after this patch:

nsh> hello
Hello, World!!
[    1.130000] Hello, World!!
nsh> hello
Hello, World!!
[    2.420000] Hello, World!!
nsh> 

[anchao]

cc @wangchdo