Rename __pure, __pure2, and __purefunc.

These names were pretty misleading (aka "backwards"), so switch to the
same obvious names glibc uses.

Test: build.
Change-Id: Ia98c9dbbccd0820386116562347654e84669034a
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 4e7d8ba..130ede6 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -81,7 +81,7 @@
 pid_t  fork(void);
 pid_t  vfork(void);
 pid_t  getpid(void);
-pid_t  gettid(void) __pure2;
+pid_t  gettid(void) __attribute_const__;
 pid_t  getpgid(pid_t __pid);
 int    setpgid(pid_t __pid, pid_t __pgid);
 pid_t  getppid(void);