um: Use asm-generic/timex.h over the host architecture one

The removal of the architecture-specific asm/timex.h was meant to fall back to
asm-generic/timex.h. However on UML, the host architecture headers are earlier
on the include path and therefore asm/timex.h from the host architecture is
used.

On x86 that header in turn requires the generation of cpufeaturemasks.h, which
is not set up for UML and the build fails.

  In file included from ../arch/x86/include/asm/tsc.h:9,
                   from ../arch/x86/include/asm/timex.h:6,
                   from ../include/linux/timex.h:67,
                   from ../include/linux/time32.h:13,
                   from ../include/linux/time.h:60,
                   from ../include/linux/jiffies.h:10,
                   from ../include/linux/ktime.h:25,
                   from ../include/linux/timer.h:6,
                   from ../include/linux/uprobes.h:18,
                   from ../include/linux/mm_types.h:16,
                   from ../include/linux/mmzone.h:22,
                   from ../include/linux/gfp.h:7,
                   from ../include/linux/slab.h:17,
                   from ../include/linux/crypto.h:18,
                   from ../arch/um/kernel/asm-offsets.c:6:
  arch/x86/include/asm/tsc.h: In function ‘get_cycles’:
  arch/um/include/asm/cpufeature.h:52:39: error: implicit declaration of function ‘DISABLED_MASK_BIT_SET’ [-Wimplicit-function-declaration]
     52 |         (__builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : _static_cpu_has(bit))
        |                                       ^~~~~~~~~~~~~~~~~~~~~
  arch/x86/include/asm/tsc.h:79:14: note: in expansion of macro ‘cpu_feature_enabled’
     79 |         if (!cpu_feature_enabled(X86_FEATURE_TSC))
        |              ^~~~~~~~~~~~~~~~~~~
  arch/um/include/asm/cpufeature.h:115:17: error: implicit declaration of function ‘_static_cpu_has’ [-Wimplicit-function-declaration]
    115 |                 _static_cpu_has(bit)                            \
        |                 ^~~~~~~~~~~~~~~
  ...

The x86-specific timex.h also doesn't make sense on UML in general.

Reintroduce the UML-specific asm/timex.h and add a comment to document
its necessity.

  [ bp: Improve that comment to actually state why stuff is there. ]

Fixes: 3ed403bbc9 ("treewide: Remove CLOCK_TICK_RATE")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: David Gow <david@davidgow.net>
Link: https://patch.msgid.link/20260819-uml-timex-fix-v1-1-f690b514fc9f@linutronix.de

arch/um/include/asm/timex.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __UM_TIMEX_H
+#define __UM_TIMEX_H
+
+/*
+ * Do not fall back to the host architecture header because latter likely
+ * includes facilities like cpu_feature_enabled() which are present only
+ * there. That would result in build breakages and/or efforts to "emulate"
+ * those facilities in UML.
+ */
+#include <asm-generic/timex.h>
+
+#endif