Fix #123950: Add missing HAVE_CLOCK_MONOTONIC to config.h.in (#124086)

Fixes issue #123950 where Task.Delay freezes when system time jumps
backwards on .NET 10 Linux - a regression from .NET 8 and 9.

The root cause was a missing `#cmakedefine01 HAVE_CLOCK_MONOTONIC`
directive in `src/coreclr/pal/src/config.h.in`. While CMake was
successfully detecting monotonic clock support, the configuration value
was never propagated to the generated config.h header. This caused the
runtime to fall back to wall-clock time (CLOCK_REALTIME) instead of
monotonic clocks (CLOCK_MONOTONIC), making timing operations vulnerable
to system time adjustments.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: Copilot

src/coreclr/pal/src/config.h.in

@@ -87,6 +87,7 @@
 #cmakedefine01 HAVE_WORKING_GETTIMEOFDAY
 #cmakedefine01 HAVE_WORKING_CLOCK_GETTIME
 #cmakedefine01 HAVE_CLOCK_THREAD_CPUTIME
+#cmakedefine01 HAVE_CLOCK_MONOTONIC
 #cmakedefine01 HAVE_PTHREAD_CONDATTR_SETCLOCK
 #cmakedefine01 MMAP_ANON_IGNORES_PROTECTION
 #cmakedefine01 ONE_SHARED_MAPPING_PER_FILEREGION_PER_PROCESS