Fix Jitter time measurement on Apple platforms (#722)

On Apple platforms, Jitter RNG implementation uses `mach_absolute_time`
function to get the current value of the clock. However, this function
deals in "tick" units, not in time units. On Intel based Apple
platforms 1 tick = 1 ns, which is good enough timer resolution for
Jitter. On M1 macbooks 1 tick ~ 41.67 ns. This seems to be a problem
for Jitter since it causes frequent failures to produce enough entropy
on M1 macbooks. Another potential cause for the issue might be that
`mach_absolute_time` clock doesn't increment while they system is
asleep. I am not sure what is the definition of "asleep" on M1 macs,
how it correlates to performance and efficiency cores and different
power saving states, etc. The upstream Jitter RNG repository has
recently added support for using the system counter on `aarch64`
which seems like way to go for all 64-bit Arm platforms.

So this change:
  - Adds the system counter calls on `aarch64` platforms,
  - Removes the `__MACH__` special case.

third_party/jitterentropy/jitterentropy-base-user.h

@@ -112,15 +112,23 @@ static inline void jent_get_nstime(uint64_t *out)
 	*out = EAX_EDX_VAL(val, low, high);
 }
 
-#else /* (__x86_64__) || (__i386__) */
+#elif defined(__aarch64__)
 
 static inline void jent_get_nstime(uint64_t *out)
 {
-	/* OSX does not have clock_gettime -- taken from
-	 * http://developer.apple.com/library/mac/qa/qa1398/_index.html */
-# ifdef __MACH__
-	*out = mach_absolute_time();
-# elif _AIX
+        uint64_t ctr_val;
+        /*
+         * Use the system counter for aarch64 (64 bit ARM).
+         */
+        __asm__ volatile("mrs %0, cntvct_el0" : "=r" (ctr_val));
+        *out = ctr_val;
+}
+
+#else /* (__x86_64__) || (__i386__) || (__aarch64__) */
+
+static inline void jent_get_nstime(uint64_t *out)
+{
+#ifdef _AIX
 	/* clock_gettime() on AIX returns a timer value that increments in
 	 * steps of 1000
 	 */
@@ -131,7 +139,7 @@ static inline void jent_get_nstime(uint64_t *out)
 	tmp = tmp << 32;
 	tmp = tmp | aixtime.tb_low;
 	*out = tmp;
-# else /* __MACH__ */
+# else /* __AIX */
 	/* we could use CLOCK_MONOTONIC(_RAW), but with CLOCK_REALTIME
 	 * we get some nice extra entropy once in a while from the NTP actions
 	 * that we want to use as well... though, we do not rely on that
@@ -144,10 +152,10 @@ static inline void jent_get_nstime(uint64_t *out)
 		tmp = tmp + (uint64_t)time.tv_nsec;
 	}
 	*out = tmp;
-# endif /* __MACH__ */
+# endif /* __AIX */
 }
 
-#endif /* __x86_64__ */
+#endif /* (__x86_64__) || (__i386__) || (__aarch64__) */
 
 static inline void *jent_zalloc(size_t len)
 {