target-alpha: Add high-resolution access to wall clock and an alarm.

The alarm is a fully general one-shot time comparator, which will be
usable under Linux as a hrtimer source.  It's much more flexible than
the RTC source available on real hardware.

The wall clock allows the guest access to the host timekeeping.  Much
like the KVM wall clock source for other guests.

Both are accessed via the PALcode Cserve entry point.

Signed-off-by: Richard Henderson <rth@twiddle.net>

target-alpha/cpu.h

@@ -265,6 +265,10 @@ struct CPUAlphaState {
     uint64_t scratch[24];
 #endif
 
+    /* This alarm doesn't exist in real hardware; we wish it did.  */
+    struct QEMUTimer *alarm_timer;
+    uint64_t alarm_expire;
+
 #if TARGET_LONG_BITS > HOST_LONG_BITS
     /* temporary fixed-point registers
      * used to emulate 64 bits target on 32 bits hosts