use CLOCK_BOOTTIME in Instant::now

There was a discussion on #87907 that all platforms expect
linux advance its monotonic timer if the system goes into a sleep state
(e.g. suspend or hibernate).
It was decided that CLOCK_BOOTTIME should be used on unix targets.

Author: hellow554

library/std/src/sys/unix/time.rs

@@ -291,8 +291,13 @@ mod inner {
         pub fn now() -> Instant {
             #[cfg(target_os = "macos")]
             const clock_id: libc::clockid_t = libc::CLOCK_UPTIME_RAW;
-            #[cfg(not(target_os = "macos"))]
+
+            #[cfg(target_os = "linux")]
+            const clock_id: libc::clockid_t = libc::CLOCK_BOOTTIME;
+
+            #[cfg(not(any(target_os = "macos", target_os = "linux")))]
             const clock_id: libc::clockid_t = libc::CLOCK_MONOTONIC;
+
             Instant { t: Timespec::now(clock_id) }
         }