condition: add CPUFeature

Taking a stab at implementing #14479.

Add {Condition,Assert}CPUFeature to `systemd-analyze` & friends. Implement it
by executing the CPUID instruction. Add tables for common x86/i386
features.

Tested via unit tests + checked that commands such as:

```bash
systemd-analyze condition 'AssertCPUFeature = rdrand'
```

Succeed as expected and that commands such as

```bash
systemd-analyze condition 'AssertCPUFeature = foobar'
```

Fail as expected. Finally, I have amended the `systemd.unit` manual page
with the new condition and the list of all currently supported flags.

src/test/test-condition.c

@@ -439,6 +439,27 @@ static void test_condition_test_kernel_version(void) {
         condition_free(condition);
 }
 
+#if defined(__i386__) || defined(__x86_64__)
+static void test_condition_test_cpufeature(void) {
+        Condition *condition;
+
+        condition = condition_new(CONDITION_CPU_FEATURE, "fpu", false, false);
+        assert_se(condition);
+        assert_se(condition_test(condition, environ) > 0);
+        condition_free(condition);
+
+        condition = condition_new(CONDITION_CPU_FEATURE, "somecpufeaturethatreallydoesntmakesense", false, false);
+        assert_se(condition);
+        assert_se(condition_test(condition, environ) == 0);
+        condition_free(condition);
+
+        condition = condition_new(CONDITION_CPU_FEATURE, "a", false, false);
+        assert_se(condition);
+        assert_se(condition_test(condition, environ) == 0);
+        condition_free(condition);
+}
+#endif
+
 static void test_condition_test_security(void) {
         Condition *condition;
 
@@ -864,6 +885,9 @@ int main(int argc, char *argv[]) {
         test_condition_test_cpus();
         test_condition_test_memory();
         test_condition_test_environment();
+#if defined(__i386__) || defined(__x86_64__)
+        test_condition_test_cpufeature();
+#endif
 
         return 0;
 }