Stop using the __ANDROID_API_x__ constants.

Historically we've made a few mistakes where they haven't matched the
right number. And most non-Googlers are much more familiar with the
numbers, so it seems to make sense to rely more on them. Especially in
header files, which we actually expect real people to have to read from
time to time.

Test: treehugger
Change-Id: I0d4a97454ee108de1d32f21df285315c5488d886
diff --git a/libc/arch-common/bionic/pthread_atfork.h b/libc/arch-common/bionic/pthread_atfork.h
index 742d078..02e383d 100644
--- a/libc/arch-common/bionic/pthread_atfork.h
+++ b/libc/arch-common/bionic/pthread_atfork.h
@@ -18,7 +18,7 @@
 
 // __register_atfork wasn't available until android-23. When using libc.a, we're
 // using the latest library regardless of target API level.
-#if defined(_FORCE_CRT_ATFORK) || __ANDROID_API__ >= __ANDROID_API_M__
+#if defined(_FORCE_CRT_ATFORK) || __ANDROID_API__ >= 23
 
 extern void* __dso_handle;
 
@@ -33,4 +33,4 @@
   return __register_atfork(prepare, parent, child, &__dso_handle);
 }
 
-#endif  /* __ANDROID_API__ >= __ANDROID_API_M__ */
+#endif